PW: Shield is pretty decent, if you're a Disc healer.
You can make it a lot better than the default spell - have it heal for a small amount, protect for a lot more, damage opponents as they break through it (threat free), and give you fairly substantial haste on your next cast.
And it is instant cast, so less mana, etc.
Fits in with the whole idea of mitigating income damage instead of healing after the damage has been dealt.

For Holy, it is another tool in the arsenal.
But much less useful.