Honestly it's a weird mentality from vanilla WoW, and probably beyond, where certain classes have the "privilege" of higher utlity and thus have a "hybrid tax", the hybrid of support (heals, buffs, what-have-you) and DPS, rather than a "pure" DPS with little to no utility. Reason is because the utility is an "advantage" in survival, etc..
However I don't know WHY this mindset in developers ever happened...