I have always been an advocate of bringing cloths for all occasions. I think the first time I realized that you needed to have an avoidance set as well as a stam set was fighting Prince in Karazhan. While having an hp suit for progression is often preferred you have to look at the mechanics of the boss that you are fighting and make a decision based on said mechanics. Prince for example had an extremely high swing rate. IMO there was a significant disadvantage in simply having a boatload of stam. You take constant quick shots from Prince and what it boils down to, is that your heal core can stay ahead of the damage output Prince is laying down. If heals in do not exceed or equal damage out by Prince your raid will fail. Simply piling on more hp doesn't always promise a positive result in this instance. Avoidance however, the ability to completely not take damage, is extremely useful especially considering you take a debuff that increases damage done to you as a tank. That debuff does NOTHING if you aren't even taking damage.
On the opposite side of the coin, bosses that hit like trucks with a slightly slower swing rate, require a stam advantage. It becomes more imparitive for you to be able to survive back to back hits, then it is for you to simply not get hit. The thinking behind this, imo, is that RNG is exactly what it says it is. Random. Even though your avoidance is 60%, there is a 40% chance for you to take a hit on one swing and again a second time (the statistics are actually completely wrong in this comment but I'd like to leave this post brief). The amount of uncertainty built into taking 2 back to back shots is high enough to make a stam set preferable.
I also have some thoughts and opinions about how and avoidance set might be better if you have a weak heal core but I think that's a conversation for another thread.
Execute for show, tank for dough.