Everything is situational, that is the beauty of tanking knowing when to go balls to the wall threat, knowing when to be ready to move or throw down burst threat. So yes if threat is that tight then yeah focus on that, but knowing when to do that is pretty much what you need to be doing anyway. Spamming 969 without understanding that is just poor gaming, and although it should go without saying I think it has to be said every now and then.
I still don't see a reason when you won't self-cleanse, it's one cooldown, and if threat is that tight then ether the encounter is designed to be a threat race to begin with or thee are other mechanics at work that take precedence over threat such as movement and the like.
But with that said, it comes down to one's judgment and making a decision. Which is why I love tanking to begin with.
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I am fucking awesome.