First I have been 25 man raiding disc, and have seen no problems what so ever. The only thing lacking is a non party group heal, which will change with PoH in 3.1. Every class and spec in game right now brings some unique abilities to the table, putting them together is what makes for a better more diverse raid able to handle near any situation that should arise.
The one thing i see missing in this thread and ignored by some cocky trolls, is the pure awesomeness of penanace. With moderate gear right now, my penance ticks for around 3.5k each tick= 10.5k healed in under a second. This also gives all 3 stacks of grace. Crit ticks range from 4.5k-6.5k, and proc divine aegis. Add in borrowed time and/or trinket procs, crits etc penance can heal around 15k in .5 seconds or less. Talented its on an 8 second cooldown, but anyone disputing Disc validity really can't ignore the importance of that kind of heal. I'll be the first to admit penance is a top spot for over healing, but raid healing right now is usually reduced to shield and possibly renew someone taking light damage, flash heals where needed, and throwing around penance near every time its up for any instant or big heals needed. It's also superior to a gheal when damage is incoming, so on a tank it keeps them topped off without to much over heal if timed properly.
Previously disc was not a PvE healing build, but anyone that still believes that must not be playing wrath. Sure pallys and disc priests might serve near the same role, but a pally isn't always available nor would you stack 5 pallies just because of one of their strengths. Add to this near every BC holy pally I know has since swtiched to ret or prot, from what i've been told pally healing is /yawn now. Not that I don't have the same feeling at times, but priests just have a bigger bag of tricks to play with. Right now my guild has only 1 raiding holy pally, that can't make all raids. We typically run with Druid, holy priest, disc priest, shaman and then usually who ever else is available.
In general healing right now is exteremly easy so cutting any *skilled* diverse healing class will only hurt you in the long run. It's been already said Uldar will be tuned assuming everyone is in full T7, so I really would like to talk to you people when 3.1 hits and you are saying where are all of the good geared disc priests and see they are happy in the guilds they are already in. It was the same reception for shadow priests in BC, no one wanted them due to past conceptions then so many people complained about not being able to find good shadow priests because they were all already in guilds that moved past preconceptions and saw the utility they brought to the raid.