For WPvP you don't want to spend a lot of time reacting. I found when I was driving as healer that I could not be in reactive mode. Everything had to be proactive. Setups and being able to lean on a mechanism like fear for example. Feared is doing no damage so no need to heal.

Anyway that'd be the argument against using a healer. You could totally go wPVP without one. Personally I prefer one but I haven't touched wPvP at all this expansion so maybe that's why. FYI I switched from click healing to hotkey healing to make my healer somewhat competent in wPvP years ago. Like WoD era.