From Ask Creative Development -- Round II Q & A 23 Jun 2011 (
http://us.battle.net/wow/en/forum/topic/2721372142) in World of Warcraft this has been changed to be that "Channeling the Light in any way, or receiving healing from the Light, only causes pain. Forsaken priests do not disintegrate or explode from channeling the Light." However, following up that question it would seem that channelling the Light has caused some Forsaken to slowly experience a sharpening of their dulled senses. So in addition to pain they also experience the decay of their flesh more acutely.
Even if they use paladin like abilities they arent true paladins.. And they will get destroyed by that power..