I cut out the bulk of the quote, hope you don't mind.
Originally Posted by Bullah
Part of the problem is what is the bang for the buck on stamina gems? Figure, even if you slap in +67 (I don't remember the exact amount for the rare stam gem) stam gems in every single spot on a fair set of gear, you're looking at somewhere between 600 and 800 stam, depending on the gear. That only balances out to 8k health, at best (more if you're a druid). ONE stam trinket can make up the difference on that. Plus, when you get to the nitty gritty, on a tank with t13 gear.... that 8k health (maybe 10k buffed) is MAYBE 5% of their health total.
Next, consider the stat growth on gear. What's the stam increase on a single set of t13 gear versus a t12 piece? Last I checked, on major set pieces, it was around 100 stam each. Gem all you want, but you'll never make up the difference that you'll gain simply on upgrading the basic gear.
Lastly, what tools has WoW given us as players to really do much about magic damage? Pally buff.... priest buff.... druid buff.... all 3 can give an increase to resistance that's hard to beat. I FREQUENTLY would nag on our pallies in t12 and tell them to turn off their stupid ret aura in Firelands as.... the paltry 121 damage it did back to enemies was NOTHING compared ot the blanket 24.5% fire damage reducetion resist aura granted EVERYONE. But beyond that.... there's really little else a tank can do. Sure... we have some cooldowns that we can use to reduce momentary damage, but there are really no other mechanisms beyond stamina. Magic damage.... you almost have to accept that your healer is going to have to heal through it.
That's why you see in general, "gem for CTC/mastery until cap.... then gem for stam". Because with mastery/dodge/parry, we can influence at least one variable we have to account for. Going by your own numbers above..... what is the greater portion of your damage? Sure... magic is a solid 35%, but that means gearing properly, you can heavily influence the 65% or more that you take from physical. THAT.... is still the larger portion. I mean, you have to consider this as part of a tank/heals team. Which one will your healer be able to keep you up through?
A) Taking at least 30% less physical damage guaranteed (CTC capped) and taking full magic damage
B) Taking some full shots from physical damage and taking full magic damage. Always.
Which one will your healer more likely be able to keep you up through?
This has been at the heart of tanking for all of Cata. The tank is going to take a heavy beating..... how can we best smooth out the damage spikes so that healers can keep up? Large, repeated damage spikes overwhelmed healers early on in Cata. That improved later, but the issue really comes back to one basic idea.....build the most stable/steady tank you can. Avoid successive, large damage spikes.
For pally/warrior.... this is done through CTC largely. For DK.... this is done by wisely using death strike. Druids.... savage defense (though, their armor nearly makes up the difference, plus large health pool).
That a bit of an explanation for the reason behind why we do that. Make sense?
No one tanks in a void.........