You have five slots. Rampart is hands down the best ability, Provoke is a necessity, and Convalescence is just too good to pass off. So you're left with two slots and tasked with picking between Awareness, Reprisal, and... Shirk.

Between nullifying critical hits and making tank swaps easier, I'd say a 10% damage mitigation wins.

Now Awareness isn't actually a proper cooldown, unless you know beforehand you'll be hit with a crit, and that only applies in certain fights.

So I can understand why people might take Shirk over Awareness, but it is NOT better than anything else. Does it make swapping easier? Of course, but it is by no means needed to pull off a tank-swap successfully. Is it a requirement? Definitely not. But I can already see that's not going to stop certain tanks from rage-quitting parties the second they find out the other tank didn't bring Shirk. Like yeah, maybe it'll be better than Awareness THERE, but it's NOT something anyone should be whining about. -_-