Gravity over at pwnwear mused some time ago about why so many tanks are into theory. I've been thinking about that on and off and there are two reasons which occur to me.
Firstly, if you are a tank then a big part of your job is to not take damage (that's not all of your job, but it's a big part of it). How can you get better at this? You can make sure that you keep up one or two debuffs on the boss, and one or two buffs on yourself, but that is so basic as to be assumed (ie you are a bad tank if you don't do this, but not necssarily a good tank if you do). You can use one or two reactive abilities such as interrupts, or cooldowns to coincide with periods of increased boss damage, but that has only situational effect. Otherwise, you just can't get better at this part of your job by improving your rotations, experience or skill.
You ensure that you take less damage by gearing appropriately, and the fact you need to gear properly means that good tanks take a healthy interest in the effect that gear has on their ability to do their job.
The second reason is that, refreshingly, there is no right answer. When I spec fury there is a "right" spec to use, and there are "right" answers for what gear I should equip (strength vs armour pen vs crit, for example has a right answer). For tanks there are no right answers - it is unarguable for example that a tank who stacks avoidance will take less damage than a tank who stacks stamina. But is this the right thing to do? Everyone has their own opinion, and the considerations change depending on class, role, and encounter.
Theorycrafting for dps specs is very broadly a case of - insert your gear into the spreadsheet, read the optimum rotation on EJ, rinse repeat. The work has been done by clever people already and you just need to research the best approach. Any theorycrafting you do is simply an inferior version of the work done by someone else. Theorycrafting for tanks is an organic community, and everyone's work can make a difference without ever being the final word on the subject.
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