Your assuming a situation with only one offtank, though. There's several fights this expansion that are literally impossible without at least 3 tanks--Eireen hardmode for example. Most raidforces are not going to be running 3 dirges, 3 coercers, and 3 assassins/swashies. A paladin can be right there in the mage group with a troub and an illy for utility instead of dirge/coercer, have amends on the warlock, and keep those adds with cooperative strike from murderizing the main tank. Two tanks are absolutely required for most fights, and a great many become easier with a third. In a three-tank paradigm, a paladin's flexibility is quite desireable.
But you're right--amends puts you too near the hate transfer cap to be quite as nice when you have a good group setup. I think the hate transfer cap needs to be raised to 100% so that a paladin has an aggro advantage in every situation. With consistent aggro on lockdown, the paladin's weakness of not having very many snaps becomes a reasonable tradeoff instead of a serious deficiency. Or they could alter Holy Ground as others have suggested. Or they could give us more content that needs 3 tanks, so that there's always a place for a crusader in the mage group. I think that that is one of the absolute best options out there for helping raiding tanks, and they've quite clearly been making a serious effort to do that. Cooperative Strike, terrorstruck, tank adds that can only be killed by certain classes...they've done a good job of making interesting scripts that will encourage the existence of a 3rd tank.