In fact, we'd arrived at a place where melee in general just had too much up-time on casters, especially healers. Conditions were prompting a move toward more instant cast spells and casting on the move, when we wanted to take PvP back to a place where cast time spells could still have a place on the battlefield.
Going to continue in a second post since the first one got long...
I also feel that the first sentence about melee uptime on healers is wrong. Pallies have freedom+sprint (out of LoS), druids can shift snares outside of desecration and priests can now suppress snares with desecration. The only healer that HAS to tank melee is a shaman since you can't heal for anything in ghost wolf. That said a shaman can kite quite a few melee classes if there is only 1 of them: Warriors, rogues and DK can be easily kited, ferals and rets cannot. With the assistance of snares and line of sight, any healer can kite (quite a while) a warrior, rogue or DK. Rets and ferals are different in that they can remove snares but while being attacked it is extremley hard for both to stay on a target since they will eventually run OOM from removing them (or can't remove them fast enough).
As for casters not using spells with cast times in PvP, you should prolly focus mainly on mages and warlocks. Shadow priests use lots of cast time spells and still do fine.
For mages, ice lance got buffed so much that the damage gained from casting frost bolt over spamming ice lance is generally not worth the reward. One major complaint from mages is that they don't feel rewarded for getting casted spells off. Part of this was because of the push to get frost PvE to be viable and use ice lance, but the risk/reward is off.
For warlocks (affliction), glyph of soul swap means they don't really have to cast much at all. Previously if a lock could keep UA+haunt on one target and UA on a second (with other dots on both), they were rewarded with pretty good damage. Now, they really only have to cast UA once and just swap it to the other target. Once they have their dots up, none of their cast time spells have that much of a reward for getting them off. If UA had its damage increased (and lowered somewhere else) and soul swap glyph got changed (nerfed) somehow, then locks would likely cast more.
In short, its not the overall state of melee uptime that was promoting the shift towards instant cast spells, it was the risk:reward was off from class mechanics. If casting was as rewarding as it was in wrath then I think that would solve the problem.
This is ignoring a bunch of non-viable hybrid caster specs, but they have lots of other problems which are not related that hurt them more than this.