 team is also extremely malleable.
With that said, lets go over the general things you can gain from each profession as a secondary!
If you choose Warrior as your secondary profession, you'll be able to effectively use any of the three Warrior weapons: sword, hammer, or axe. However, these are all melee weapons, so caster primary professions, with their relatively low durability, should be cautious about actually trying to use these weapons in combat, especially as the game progresses.
Warrior does have a number of useful defensive and party support skills in the Tactics attribute line. These can be useful for anyone and are the most likely skills to be useful in a general, augmenting sense. You can also get shields that require Tactics, which become more effective if you invest in the Tactics attribute line. It is perfectly reasonable to have a caster that uses a shield and scepter to be more defensive, though I wouldn't want to invest in Tactics just to use a shield more effectively and not bring at least one Tactics skill.
I do want to point out that Sprint, a Strength skill that increases movement speed, does have a decent duration even without any investment into Strength (the Warrior's primary attribute line).
Overall, I recommend Warrior as a secondary for other physical professions more than I do for caster professions, since so many of its skills are centered around using melee weapons. Having access to these weapons to change up the gameplay when using another physical profession can also be a benefit, depending upon your playstyle.
If you choose the Ranger as your secondary profession, it will enable you to effectively use bows via the Marksmanship attribute, though do note that bow attacks tend to have relatively high energy costs for attack skills, which can make them harder to use for some professions such as Warrior or Paragon.
Choosing Ranger as your secondary also gives you access to Beast Mastery, which will enable you to bring a pet to fight with you.
Wilderness Survival also has a number of useful defensive and support skills, though like most Ranger skills, energy costs are balanced around the Ranger's primary attribute, Expertise, which lowers the cost of the Ranger's skills, making them potentially too costly for other primaries to use.
In my opinion, the two biggest draws to choosing Ranger as your secondary are Beast Mastery and Antidote Signet. The latter of these is a powerful skill for removing conditions from yourself and is especially useful for physical attackers. This may sound like a small thing, but given how effective it is, how low its costs are, and the fact that is a no attribute skill (meaning it doesn't require attribute point investment), it is actually very useful.
Overall, I recommend Ranger as a secondary profession for those who want to play with pets.
A Necromancer secondary is most useful to other casters, though Blood Renewal, a self-healing skill, is useful to everyone early on. Additionally, there are a few skills—Plague Touch and Plague Sending, specifically—that transfer conditions from you to enemies (note that these skills work perfectly fine without investing any attribute points in Curses, though they do benefit from even a relatively minimal investment). These can be quite useful. Playing with minions can also be fun.
However, I generally wouldn't recommend Necromancer as a starting secondary unless you have a specific flavorful conceit you're developing. This is because Necromancer, as a profession, is somewhat lacking in general utility compared with other professions. It isn't a bad choice, to be clear, but other professions will often provide you with more utility choices than the Necromancer will. There are some neat builds you can do with a Necromancer secondary, but most of them require skills you won't get until after you can change secondary professions, anyway. Note that none of this is meant to imply that Necromancer is a bad primary profession; it decidedly isn't!
Overall, I recommend Necromancer as a secondary profession if you want the flavor or feel of it. Otherwise, you've generally got better mechanical options elsewhere, particularly for your initial choice.
While Monk is primarily a healing profession, you generally shouldn't take it as a secondary for that purpose. This is because most of the Monk's healing skills (generally found in Healing Prayers) are balanced to be a little on the weak side without the benefit of the Monk's primary attribute, Divine Favor, which gives additional healing to Monk spells that target allies.
That said, Monk can still be a great secondary profession. It has a lot of useful support enchantments, especially in Protection Prayers, which can be used to defend allies. It also has skills to cure both hexes and conditions, the two types of debuffs in the game. This can be extremely useful!
Another extremely valuable type of skill that the Monk has are resurrection skills. While every character can get access to the Resurrection Signet skill, that skill only recharges when you get a Morale Boost, which can be an infrequent event. The Monk resurrection skills, on the other hand, have normal recharge times. This can be extremely useful, especially when playing with Henchmen, who often only use Resurrection Signet.
Overall, I recommend Monk for those who want to support their party more broadly, that want to be able to remove both hexes and conditions from themselves or others, or that don't have a particular inclination towards any of the professions as a secondary so that they can have access to the Monk's resurrection skills.
Elementalist has a lot to offer as a secondary profession, as it has a lot of useful utility skills available. However, I'd caution against taking Elementalist as a secondary for its offensive capabilities—if that's something you want to do, I recommend playing a primary Elementalist, as the ability to use runes helps tremendously in this regard. Not that you can't try to do this, but do note that attempting to make meaningful use of just a couple of Elementalist damage spells can be quite challenging to accomplish from a build-craft perspective, particularly from an energy management standpoint.
Additionally, Air Magic has many useful skills for inflicting conditions such as Blind, Weakness, or Cracked Armor; Water Magic has many useful snares; and Earth Magic has a lot of useful defensive skills.
Also, Glyph of Lesser Energy, a skill in the Energy Storage primary attribute, is nevertheless useful for all casters as a non-elite form of energy management. It is most likely to be useful for Necromancer and Mesmer primaries (as they have somewhat higher energy costs on average among casters), but all casters will be able to get benefit from it.
Overall, I recommend Elementalist as a secondary profession if you want offensive utility or to augment melee attackers with useful support skills.
Mesmer is an extremely useful secondary profession owing to its wide array of utility skills. This includes spell-based interrupts, hex removal, enchantment removal, snares, energy management, and damage reduction.
There are numerous advantages to spell-based interrupts over attack-based interrupts. They don't require you to be next to the target (unlike melee attack interrupts), don't have flight time (unlike bows), and can't be blocked or miss (unlike all attack-based interrupts). However, interrupting well is a more advanced skill, in that doing so requires the ability to recognize skills in order to prioritize the use of interrupts effectively. It also takes practice—I know the skills in the game extremely well, but it still takes me a while to warm up when it comes to interrupting. Still, it is a useful (and fun!) player skill to add to your repertoire.
Hex removal is also often useful, and it isn't something many professions actually have access to. The Monk has hex removal, and the Dervish has a couple of skills it can use to remove hexes from itself. And, technically, the Paragon has a hex removal skill, but it's clunky to use. So while it can be argued that a couple of the Monk hex removal skills are preferable, the Mesmer ones are most definitely useful when taken as part of the overall Mesmer package, which includes many things the Monk can't do.
Which brings us to enchantment removal. This is something the Mesmer is fairly good at, and multiple options are available, including some enchantment removal skills that do additional damage or work as energy management. These skills can be notably useful for physical attackers, who often want to be able to remove defensive enchantments (though note that this can be done by any party member).
Snares are also very useful. These are skills that slow enemy movement. Snared melee enemies have a harder time damaging your party members, while snared enemies in general have a harder time evading your own melee attackers. This makes snares useful both defensively and offensively, and in a way that can be subtle and easy to underrate.
A major reason for other caster professions to choose a Mesmer secondary are the wide assortment of energy management options available in Inspiration Magic. A couple of the more powerful ones are interrupts, and therefore somewhat more difficult to use, but nevertheless, having access to solid, non-elite energy management skills can be extremely valuable. Note that the interrupt-based energy management skills are notably effective on Monk heroes. There are also several elite energy management options that are pretty solid.
Finally, it is worth mentioning some of the damage mitigating options, notably the Mantras of Frost, Flame, Earth, and Lightning. These st