Originally Posted by
Xequecal
The difference is, on a boss you have to taunt swap, Brewmasters can charge up their Guard and EB charges while they're not being hit, no other tank can charge up their active mitigation while they're not being hit to anywhere near of an extent. Sure the other tanks can max out rage/holy power/blood shield while they're not tanking but that effect is small and will get blown through very quickly once they're taking hits again.
Even in Mythic I doubt there are bosses that can deal 2.5m damage to you in 30 seconds when you start with 15 EB charges, which is how much Guard you will have assuming you taunt swap the boss every 30.