There are videos of a 4-boxer only playing disc priests on youtube.
He plays instances in lfg with a random tank only dps with the 4 disc priests while Atonement heals the group and the tank.
Sometimes he puts a renew or a shield on the tank when there is heavy damage incoming but most of the time he only DPS.

I don't know if he can get a solid DPS with a disc priest but he doesn't need to focus on heals and he got 4xDPS instead of 3xDPS and 1xHeal with 0 DPS.