Are gates too slow for constructed play?
Short answer: unconditionally entering the battlefield tapped leaves them at too great a disadvantage with respect to the other options, and these lands are designed for limited play.

More specifically, there's essentially no advantage to using Gates over the corresponding shocklands or core set/Innistrad duals; since there's virtually no land type specific LD, gates are for all practical purposes strictly worse than their corresponding shocklands, because the shocklands can be fetched with Farseek and otherwise have the option of coming into play untapped if you're willing to take the hit.  Likewise, the core/Inn lands are exactly like the Gates except for the possibility of coming into play untapped on turn 2+ (and note that the shocklands 'turn on' the core lands, which is another point in shocklands' favor).

Between these two factors, the only thing that Gatelands have to offer over the other choices is that little word: 'Gate'.  If there were cards that took real advantage of the gate type - say, something like a Rampant Growth/Farseek for Gates - then I think you would actually see them getting some play in ramp decks.  As it is, though, the only cards that interact with Gates are a couple of creatures (none of which is within constructed-stats range) that gain small benefits for gates in play and Gatecreeper Vine, which fetches a gate but - unfortunately - doesn't put it into play.  This means that the land type (at least for now) isn't offering any upside to offset the advantages that the other styles of dual have.

So why do they exist?  Well, pretty much for the reason that you noted; RTR limited is going to need quite a bit of manafixing since there are so many multicolor cards; this means putting fixing down into the common slots.  And while Rarity Is Not Power, WotC has a number of reasons for not having rares 'outclassed' in substantial ways by commons, and they've done very few powerful land cycles at common (or even uncommon) - you can generally expect that any land-based mana fixing that winds up in the common slot is going to be a little bit weak.