Originally Posted by Astrylian

Many bosses can melee while casting spells; it depends on the spell, so that's a very valid question. I don't know whether Shear is one of those spells.

Also, for bosses that don't hit exceptionally hard (pretty much everything up to Kaz'rogal or Teron, with the exception of Tidewalker), stacking avoidance is perfectly reasonable, because you can get enough survivability from raid buffs at that point.