Actually, disc priests are GREAT for PvE. They are incredibly good tank healers, and can throw around shields on other people if the tank damage isn't too rough. Shields are awesome because they effectively allow preemptive healing.
For the record, I do not play a priest of any kind, I just happen to do my research rather than look at recount and say "OMG, teh disc priests HPS is soo low. they are pvp spec, gtfo."

Agreed with Adrian. But you won't get it until you learn how to use disc effectively and then try it for yourself.

I'm in a 25-man raiding guild as a healing priest. In 25s, I'm always Holy, almost without exception. But in our 10s that we run on off nights, while I'm almost always there, other healers are more fluid. Sometimes I end up with two pallies with me, so I'll stay Holy. Other times, I'll end up teamed up with a shaman and druid and will go Disc to cover tank heals.

Until you're the one tossing out shields on the entire raid and noticing for yourself how much difference they make in raidwide damage that would normally need to be healed through, you can't possibly get how powerful disc is. I know I didn't.