Sorry for continuing this off-topic, but i'm a lover of MDPS Shaman, i played a lot of 10v10 and 3v3 here as Shaman halfkin and its is so fun but this works 10% of the times as mdps and 5% as healer.
As Healer:
As you said the main problem is that it has no PW, beyond that, Totems are kinda useless, dies too fast, the effects has a small chance of success. Callback is op but too high cooldown time. 20% Cast Speed using Staff is useless here because if you gonna heal, you must use mace/shield and the only spammable skills is the AOE Heal and lightning DoT.
As MDPS
Natural Punishment is not spammable but the glyph makes this usable. That's all. Lightning DoT is useless in PvP (btw you have to cast it and does no base dmg, only procs dot). After the chages in Tornado (can i call it a nerf? and a huge nerf for bard/shaman halfkins) we cant use it to deal dmg when natural pushment is on cooldown.
Shaman can do alot of things, but it always there's other classes doing the same but way better.
If you want to we can create a new topic for shaman suggestions.