mrmr wrote:keyword here? "Block"....
Keywords? "Armor", "HP pool", "multi-role".
Yes a prot warrior will have an overall better damage mitigation than a druid, using "no-brain theorycrafting".
I say "no brain" because it is all about making false assumption like neglecting magic attacks, neglecting the risk to die, flat statistics over procs.
To achieve the best mean damage mitigation, a warrior rely on procs. To block and to parry are "procs". It can happen you take 3 blow in a raw with no block or parry. Such a case is often catastrophic for the warrior.(Don't tell me a well geared prot warrior is always either blocking or parrying or dodging, it is again theory while in practice, the tank is stun, bumped, and makes mistakes.)
In the meantime, the druid has less "procs" (no block or parry), but a better mitigation on direct hits (higher armor). Moreover, the druid has more HPs, which means it is less likely to die within a very short amount of time.
A good example is Lashlayer in BWL. On retail, my guild very quickly understood the strength of a druid tank on such a boss. His mortal strike can one shoot a warrior in case of crit. If ever the warrior is not full life when it occurs, the death is very likely. On a good geared druid (and in BWL a good feral tank gear still includes lot of blue items), the MS hit for something like 6k while the bear has over 12k HP.
This makes a druid a very good practical solution. Even if we state (which is not sure) that the druid takes more total damage over the fight, it is less likely to die, it allows more "mistakes" from the raid, and this is what we want to win a fight!
Enough spoken of the druid as tank: another important asset is that as soon as he is not used as tank, a druid with the same spec can make a decent DPS, while the prot warrior is a useless spot. Good feral druids know how to make DPS, and at equivalent gear the DPS is the same than a rogue. I must agree however that "equivalent gear" is a condition that is rarely matched, but what is important is that the spot in the raid is used for something.