Trolls -don't- suck for certain things, actually. Don't misunderstand, the regeneration IS useless. However, the berserking and beast slaying are still pretty decent.
Trolls make pretty darn good hunters as, hey, they use bows and like haste. And hey, trolls also get Bow specialization, +1% crit with bows. I'd go as far as arguing they're potentially better hunters than orcs and their command racial.
They also make the best horde raid shaman healers, as orc and tauren bring absolutely NOTHING to shaman healer (although orcs are beasts for ele/enh shamans), whereas the haste racial -can- be useful to a healer, if situationally. For pvp, the war stomp or hardiness can be useful to a shaman healer, but then again, trolls also have Da Voodoo Shuffle for pvp.
Assassination rogues also benefit from troll racials more than orc racials, as they use daggers instead of fists or axes (though combat rogues generally are better off being orcs) and they'll benefit more from the haste than from the blood fury.
Or so I recall reading in a theorycraft once, which showed that without the expertise bonus on an orc, haste was superior to blood fury for a rogue. This might have changed admittedly, as it was quite some time back when I saw this.
Can't think of other examples, but I'm sure there are other classes/specs where troll is a good choice.
I actually find trolls to have better racials than tauren and they're plain cooler. It's just that tauren are the only horde race that can be druids, while the troll doesn't have it's own unique class to boost their population.
When trolls get to be druids in cataclysm, they'll actually be better druids than tauren, at least going by current racials. They could still get fucked over when racials get updated for cataclysm, so I guess we'll see.
Mostly, trolls are less played I think because people like to make cow/bull jokes (I mean, really, tauren suck), block people's view of mailboxes/items and the druid thing.