You cannot have warrior with doubletime+intervene+leap+ms+safeguard/shockwave or rogue with ms+bos+5x more control/peels and lolcombat dmg on top of all this and then expect any other melee class to compete for the exact same spot in the team.
It wont happen.
You can not have balance with dispel having 8 sec cd while polymorph,fear,cyclone and roots have none
You can not have druids that are completly immune to polymorph spells and all snares and on top of that give them no cd 40 yards spammable instant snare and blink and expect it to be balanced.
You can not have healers which only castable spell in 5 min game is CYCLONE
You can not have 30 sec cd priest fear with guise and feathers and expect it wont be used on your healer ON CD
Does any of those things I just mentioned are a thing in pve? No
Why isn't it fixed yet?
There are virtually zero melee caster x comps involving feral, ret, enh, DK. Each one of these classes are tied to playing with a hunter that has MS or another melee that has MS. The only one I can think of is feral/spriest that is sort of mid-tier right now. the MS change would spark many new comps. It wouldn't be so generalized where you can pick x melee and x caster, but at the very least it will open up something for ferals/rets when BM gets nerfed or will give Enh/Dks something besides TSG and Turbo.
Yes it would open new comps but giving MS effect to hybrids that can do more offheals then dps sometimes would be complete disaster. Rets,shamans,and druids simply can not have MS