Extremely unlikely. With the way WoW specializes roles so heavily - especially in high end PvE content - there's no way they'd create "half a healer" because like you said, it'd end up being totally non-viable.
For PvE, is a Monk always going to lag a little in healing to make up for the slight damage they're doing or vice versa? Like, let's say you're doing a 10 man where you'd normally want 3 healers, but a couple of your dps and one of your tanks is a monk, will you just say screw it and run 2 heals? When you look at recount, and you see that your monk healer is doing less than your other healers, do you also look at dps charts and realize that on the other hand, they're not much below your low dps, and let it slide?
For PvP, will the WW's healing be enough to keep a team strong without a dedicated healer? Will all-monk 3v3 squads become just as common as triple shammy teams? Will a dps spec with healer talents/glyphs be overly balanced?