It makes it easier to balance because tanks aren't being obliterated to compensate their insane survivability, damage can become less spiky and it also allows certain DPS classes to off tank briefly to compensate tanks needing to wait out their CD's, like they did back in TBC and WotLK, where if a tanks major cooldown was on CD and the off-tank was busy, plate dps could throw on a shield and one hander and pop their mitigation to help, rewarding coordination and proper use of cooldowns, that simply isn't possible today cause of how hard bosses hit, again tanks being near-invincible has hurt the over all health of the game and cut into the fun of other roles.
Also calling Beacon Paladin's tank healers, makes me wonder if you raided in a progression guild at all before MoP, I suspect the answer is no. But then you follow up with a claim like "there is nothing difficult about positioning." Really makes me realize you might not of tanked anything before WoD, there used to be a lot of mechanics that requires perfect dropping by tanks and small precise movements, where the slightest mistake could instantly wipe the raid.
You're rewarded by actually doing your job, which is manipulating the mobs and positioning the boss and mitigating damage.