Originally Posted by
solracht
I'll be constructive, as the other poster is probably just butthurt from hearing the word "Rift" in your suggestion.
The problem with niche tanks (or anything really) is the fact that they're useless when they're outside their niche. How would you deal with this? You either accept that and make it easy to switch to a different thing (the way Rift does, I imagine) or you give them a secondary use for when they can't perform their niche.
I don't really know if most players would be happy with either approach. Do you really want a mystic knight that you're not "allowed" to play unless the encounter is magic based? What do you do with your Paladin when that encounter comes?
You probably do want 2 tanks though, more if possible, but not limited to a niche like you're suggesting. By having several tanking classes it means you don't have to wait for a specific job. It may be harder for the developer to balance, but that's one thing they're being paid for.