I read a long time ago (can't find the quote) something on the lines of;
'Priests are the Premier healing class in World of Warcraft, this means they have two unique healing trees. Disc focuses on Single Target healing and mitigation, Holy focuses on Raid healing'
During Wrath Disc was seen as either a PVP spec or a raid bubble spec. Blizzard didn't want disc priests to just spend their entire raid popping pw:shield throughout the whole raid, unfortunately it proved to be overly effective for this, so they have changed the disc spec in Cataclysm to limit pw:shield's usefulness.. we can't just spam it anymore
Now, it seems to me that in the composite of a 25 man raid you generally take 5 healers, there are 4 healing classes.. but 5 healing specs in WoW.
Why not let Disc priests sit on par with Paladins for single target healing, and Holy, shamans and druids fill up the raid healing roles?!
Currently Disc has talents which mean I have to either take evangelism.. don't get me wrong .. its a cool talent, or throw away a talent point somewhere to avoid it.
But seriously.. past heroics.. who honestly dps's and heals at the same time.. who even wants to do that?!
I'm a pure healer who wants Disc to be as strong as holy for once. There is no reason this can't be done.