Hypothetical situation: a raid, on average, takes 40k damage per second. On this fight, skilled holy priests, holy pallies, and restro druids will be doing 14k heals/sec, while a skilled disc priest or shaman healer will only do 10.5k heals/second.
If you group the holy priest, holy pally, and resto druid together, they have a total of 42k heals/sec. You kill the boss fine.
If you group two resto shamans and a disc priest together, you have 31.5k heals/sec. You wipe.
There is a reason why Paragon and such stack holy priests and paladins.