And all those assumptions are based on what? Whirlwind is a great way of keeping the attention of the group by causing damage to 4 of them, I do a lot of stance shifting to use whatever keeps aggro the best. If a taunt fails I switch to combat and use mocking blow. Trust me this works, I rarely ever lose aggro from the attacking mobs. I do enough damage in defence stance to keep me happy and keep wearing down the mob.
Now if people wish to engage the little section of their brain that is required for actually reading (do it!). You will see I said my impression of priests is not good because of past experiences and I acknowledged they are useful if played correctly. As for Zul'farrak the only time the priest was needed was at the pyramid, and even then it was a massacre(for the mobs).
So to recap, priest = good if played correctly, crap if not. And I have been frustrated by priests screwing around too often. As for "keystone" pretty much every class is the key in a well balanced group, without a couple of tanks the instance will be over fairly quickly.
Now imagine yourself as the meatshield in a group, holding the aggro while the others wear down one at a time. Several elites beating the hell out of you....and the priest does not heal. Would that not annoy you ever so slightly?