Disc is awesome. Simple as that. It's never been in a better place than it is right now, and I love it. As a testament to that I now see other Priests who are Disc, and who aren't doing PvP. Previously I saw very, very, very few Disc Priest's that went for PvE, now I see much more.
In relation to PW:S, how can you possibly think it feels trivial?! PW:S is freaking amazing and I love it more than I can possibly say. Yes on a tank taking 15k hits it might be gone in one GCD, but that's roughly 8k damage that was absorbed instead of smashing the tank in the face. It all helps.
I've only done ToC normal so I can't speak from experience in Heroic, but we had no problems what so ever with me being Disc. I never once felt I could of contributed more by going Holy. Not once.
P3 Anub is where Disc shines in my opinion. Spam the raid with PW:S and they can absorb the damage instead of you having to heal them up and possibly heal Anub too much. No other class/spec has that sort of ability.
Since I've been Disc since early TBC I'll remain Disc until I don't want to heal anymore