"I don't know that I would say we want tanking niches. I ultimately am agreeing with what you're saying, but once we say "tanking niches" players have visions of the DK who parks outside of Icecrown until boss 4, 17 and 31 (yes, IC is that big).
I totally agree that no tank can be the worst at everything. What you have to remember though is that "everything" is generally defined as the current raid content. Coliseum isn't a large raid. If there are 5 bosses and 4 tanks then it seems that even in the best case that someone might be "the best" on 2 of them and everyone else is "the best" on 1. (And honestly, we don't design raid encounters with that goal in mind -- they have enough constraints on them already.)
Again, the goal is "close enough" and the mushiness of that definition is intended. For some groups, having a tank that is "5% harder to heal" (whatever that means) is acceptable and perhaps not even detectable. We just need to avoid the extreme situation where the guild that feels gimp because their MT is a paladin (or whatever) and paladins have a lot of liabilities on the current raid content to the extent where that player is getting replaced on more than just a couple of fights. (And I should add by knowledgeable raid leadership, and not just the Naxx pug guy that read on the forums that bears are the best tank and so only accepts bears.) "
Pas moins de 31 Boss !!
ca sera du T10 .... ca commence a faire beaucoup de set a ma banque... a quand la combinaison des anciennes pieces en un seul et unique slot ? 
