The majority of the top imp healers pvp re-rolled pub or left the game
Wrong. Sorcs we still have (in no particular order) durwin, nova, klinsmann <3, psychospider. Ops anna'ka, shatterwoman, strykez, dize, jalblurr. Mercs reighner, regis, moonrunner, amigo?...
Pug healers are depleted except for the occasional sorc but to say all the top imp healers are gone would be incorrect. It's my opinion that we are missing top end dpsers, that's why I often run dps to pick up the slack. A lot of my healer buddies still play minus Vandis but the vast majority of top dpsers on my friends list are gone. Sure there were traitors like combatmedic, amigo, okku (on vanish), neb, and especially grisha even though he doesn't heal, but there are still quite a few top imp pvp healers around. In pugs I find that imp dpsers don't focus well or peel which leaves the healers at a big disadvantage running solo. Can't tell you how many times I am the only person attacking Neb in a pug, even after I put a big target on his slippery ***.
Sorry to pick this line out but I have to respectfully disagree. To the OP- run merc we need more quality healer tanks. It is more difficult to heal as a merc in high end pvp due to properly timed interrupts/CCs but when played right they are a force to be reckoned with.