From a healer perspective, the mana blessing ret gives can't be overlooked. It's 12.5% additional mana regen and although that does not result in 12.5% more healing (as for most healers filler heals are generally their weakest) it's especially strong in long fights where starting mana and mana pot values are diminished. This is especially true if you have a disc/mw in your group who has the highest efficiency when converting excess mana to throughput.
I know most dps don't see it this way, but additional healing can be converted (at an unequal rate, usually 2:5) to damage on a fight with a tight enrage by cutting healers. Of course this highly depends on the damage pattern (i.e. vampiric embrace vs innervate better?). Bursty damage fights diminish the value of sustained healing from dps compared to sustained incoming damage where additional healing from any source is very relevant. This makes shadow priests, boomkins, rets better than their stock standard dps. Another thing that I particularly like about rets is their auto bubble, which allows healers to be more efficient healing them on bursty fights.
I'm not going to go and claim that 10% dps gap is okay, but would you take the 10% dps by giving up your blessings/durability/mobility/loh? I can tell you now survival hunters are not survivable at all, all their utility is offensive (aoe slows, roots, mobility, flare) which means nothing in NH, and all they bring is damage.
As for hc/mythic discrepancies, you guys shot yourselves in the foot by letting "Retribution" go live. Devs aren't omnipotent, they can't foresee everything and thus have to use empiric data to balance specs. Unfortunately, aggregate data which is used to balance are subject to public outcry (hunters got buffed when they didn't even need one because their hc parses were on the low end the first week of raids). The same data which is used from the first week obviously includes much higher than expected uptime on Retribution because it's the first week. On mythic, where 1 guy dying probably means a wipe... yeah you get the picture. I am thoroughly convinced that the 5-10% ret is behind on mythic is primarily caused by Retribution on hc.