Sage healing at 50 can be very frustrating since 1.2.
I can keep myself alive if I am at the very top of my game. However, the only person I am healing is myself. The instant you stop kiting or healing yourself you are dead. You can kite and bubble other people and throw out your HoT but this has limited effectivness.
Basically any single bad melee can make you ineffective/kill you.
On a good team your surviability/usefullness goes way up. That being said why any team would take a Sage healer into rateds (once they are out) is beyond me as keeping the healer alive takes too much away from actually going for the objectives.
You are are going to hear a lot of this post is just QQing but its simply the truth of how things are in PvP at 50 for Sage (and sorc) healers.
Edit to add: I will say the actual healing number are fine if you can heal, but your low surviability causes you too kite, which in turn takes away your 2 best heals.