When should I choose an Explore card in Race For The Galaxy?
Your analysis is not wrong, but it is a little simplistic, mostly because you are ignoring the existence of the other players, and the state of their games.


Explore is the only option that doesn't depend on anything else. You can always play an Explore. Yes, it's not as efficient as Trading. But Trading isn't always possible. If you've run out of cards, or your cards just plain suck, then Explore is pretty handy.
Explore prevents your opponent leeching, and taking advantage of your action. For example, an opponent playing a military strategy may be waiting for you to choose Settle, so he can dump a military world for free. Playing several Explores in a row can force opponents to choose actions they would rather you chose for them.
Similarly, you don't want to choose Settle, and settle something rubbish, if that gives your opponent the opportunity to play something amazing. So if an opponent has many cards in hand, or has a clear strategy developing based around obvious Phase choices, then you're going to want to avoid those when you can. Explore is often a good alternative.
Explore is crucial if you think an opponent is planning to Develop or Settle, and you want to take advantage of that, but don't have a card of that type in hand, or, even worse, you have a card you want to play, but don't have enough to pay for it. In this case the fact Explore happens before anything else is really valuable.
Cards that generate Explore bonuses, particularly things like Research Labs, can make Explore much more lucrative for you than for anyone else. These also allow you to leech other people's Explores. Obviously, they will try to avoid this, but as we've noted, often Explore is the only option, particularly early game, or with rash plays that have emptied one's hand unhelpfully.
In general, Explore +1/+1 makes more sense in the early game, because your hand is smaller, and later on you will usually have alternative ways to generate card flow. Explore +5 makes more sense as a later-game play, particularly to maximise your strategy. For example, finding large military worlds, getting more blue production worlds, finding that killer 6 development etc. are all things that might justify a late-game Explore.


So, overall, while Explore is often not what you want to do, it does have wide versatility, and can occasionally be fantastic. I think careful tactical use of Explore is an essential tool.