Provoke is one of those abilities that you don't understand how useful it really is until you have it. Being able to grab instant hate on ANY mob is invaluable. Even mobs that you haven't engaged yet. Yes, it doesn't work as well if you have DPS that don't know when to stop attacking, even when they've pulled hate, but it's still puts your threat at a point where you don't have to chase a mob around trying to BB or TH the crap out of it in vain. Plus, with a lot of bosses, as previously noted, it is essential due to tank swapping.
My only wish would be for them to add in an affect that would force the mob to attack you for maybe 5 seconds. Trying to pull an iron giant into position in the King Behemoth fight, for instance, is annoying even with provoke when you have DPS that want to unload immediately. You can provoke/tomahawk/run to the spot you're pulling to, but you'll still lose Aggro if a ranged won't wait until he follows you to that position.