Actually, Heather, with the addition of Focused Will, deep Discipline has become a potent (and therefore popular) PvP build. Before, many found that they lived longer relying on Blessed Resilience's crit protection. Now, deep Discipline is a rock-solid PvP build.
Deep discipline still isn't a good raiding build, regardless of the high power levels of PI and PS. Why? Lots and lots of filler talents. In order to get up to 41 Discipline and get Pain Suppression, you have to get a bunch of talents that don't help your raiding at all. In a traditional 23/38 or CoH build, no talent is wasted; you can spend every point on something that improves your healing. With deep Discipline, this is not the case.
Enlightenment and Mental Strength are expensive for what they do, and so are barely legitimate choices for raiding. Both of them combined are effectively trumped by Holy Concentration and Spirit of Redemption for half the cost. But that's not the worst of it; the worst is that you still have to take seven points' worth of talents that just aren't good for raiding. Absolution? Dispel isn't a common enough mechanic in PvE anymore. Silent Resolve? Big in 5-mans; in raids, important only in a bare few encounters. Force of Will, Imp. Mana Burn, and so many of the other mid-level Discipline talents have a single common denominator: They are not raiding talents.
All of that said: I am a raiding priest with a deep Discipline build.
The thing is, I have no illusions about it. I *know* my build isn't an ideal one for raiding. I still have my build as it is for two reasons. One, it's a compromise. I have neither the cash nor the inclination to continuously respec between PvP and PvE builds. My build sees me through alright. It's decent for both purposes. Two, it's fun. Have you ever had a whole raid's worth of casters do a roll-off for PI receptions? Ever turned an aggro-happy boomkin into an impromptu off-tank? Ever screwed around with the damage meters using PI and whispering someone "if you don't say anything I won't either"? Ever watched a boss charge at someone who's pulled aggro, use PS, and watch it turn around and go back? I use this build because it has rewards unrelated to its power that make up for its lesser effectiveness.