Wrath has an advantage of its own, it can still use divine assault and sigmars radiance strike to prop them up.
I try not to think about skills and classes in a vacuum, but rather as a whole and what their roles should be in a party. The primary role of the Warrior priest is healer, secondary role is dps. No one is bringing a melee WP as their main source of DPS, and 20s cool down on a 50% healing debuff that last 10 seconds is more then enough time to coordinate a strike on a target to secure a kill.
Possible 100% uptime of a healing debuff is far to much into the realm of the DPS classes. Class balance for damage dealing healers is tricky, and honestly order has a lot of 50% healing debuffs that are pretty easy to attain. BW/AM/SL/IB/WH
Those are the most common sources I can think of off the top of my head, I know RP and WL have 50% ones also but they are fairly uncommon.
* perhaps az, you could move the WP AoE detaunt and RP 50% heal debuff on crit around to mirror the doks dps tree more?
I can't believe I just asked for a salv WP nerf

Dunno how that could be worked out just a thought.