I think, with the hybrid tax dead and gone and some hybrids doing better dps than pure-dps classes, a 4th spec should be used to hybridize current pure dps classes.
With 4 of 11 classes being able to fulfill only one rule, and even then only one version of that role, melee or ranged dps, none can do both (except maybe Hunters could be interpreted as being both, the bastards...
) and the other classes being able to fulfill, play, not-have-to-reroll-to-play-another, 2, 3 even 4 roles I think that's the biggest thing that has to change.
Hybrid class players can just switch to other gear and specs if they want to play another role, if it's one their class offers. Yeah, gear has to be farmed but they keep their reps, personal achievements, unique stuff they got over the years, currently legendary quest progression etc. Pure DPS class players have to reroll, give up their character and all the stuff entailed if they want to switch to another role.
I played a Mage for 7 years and besides the overall suckiness of the Mage right now imho (in various regards, not - only - necessarily DPS, just class identity, spec difference, gameplay, everything - personal opinion, please don't go into it, there have been enough discussion on this) I've long wanted the option to fulfill another role in the raid. Have a use for duplicate token, have a secondary gear to build up. I still am primarily a DPS player but I'd love to have the *option*. Don't even care abuot Heal or Tank, just *something* else.