Tanking is the easiest job in the raid, so its very popular I tanked normal Incarnates using my ilvl 340 tank, sure I needed to taunt a bit more to hold aggro off the 380 warrior but overall survivability was unchanged I could take 10+ stacks from the first boss without slowing down.
Honestly there is barely enough to do in the raid for 2 tanks I cant see how people would ever justify 4.
Blizzard could scale it so 4 would be optimal at 20 man and people would still bring 2 and use more mitigation instead and keep 2 more dps