Actually, if you use Prayer of Mending on cooldown, the glyph isn't bad in fights which have low AoE damage. If your PoM is not getting all of it's jump, the glyphed version is better. Since there aren't many disc priest throughput glyphs, this one isn't a bad idea, just make sure you have tomes with you, just in case.
Maximum jumps:
100% -> 120% ->120% -> 120% -> 120% | PoM (with 2 set) 580% total (much better)
160% -> 120% -> 120% -> 120% | PoM Glyphed (with 2 set) 520% total
4 or less Jumps:
100% -> 120% ->120% -> 120% | PoM (with 2 set) 460% total
160% -> 120% -> 120% -> 120% | PoM Glyphed (with 2 set) 520% total (much better)
Without the 2 set it's a 40% less healing if PoM got all it's jumps and still 60% more healing if it did not get all of it's jumps. I use it for most, if not all, heroic encounters because, as a disc priest, our bubbles slow down the rate which they jump making it more likely that it will come off cooldown before the it gets the chance to get it's last jump or two.