My biggest thing with it is to just make sure you are still healing, and playing your role.
If you're playing your role, then who cares if you go offensive, too? But if you aren't healing anyone, and queued as a healer with the intention of doing damage (regardless of spec), then that's detrimental to the BG a lot of times.
That being said, I also find it highly silly to see so many classes do high damage output as heals. I'd be okay with one battle priest style class/spec, but the current trend is a bit odd to me and not something I'm a fan of. Then again, I also enjoy tanks in PVP. So it's all a matter of preference and opinion, really.