Originally Posted by
Danner
Until level 60-65 or so, you can heal any instance as any priest spec, shadow included. The only thing holy offers over disc or shadow is stronger heals, which is utterly unnecessary until late outland. The only thing disc offers over shadow is better tools (fast massdispel, strong shields, penance, PS, PI), which don't really matter all that much until you start seeing tanks getting all but oneshot, and fights with DPS checks.
Just make sure to get Meditation (vital for any spec), and for your own sake, dip some points into Spirit Tap for efficiency between raids. And bring water. Lots of water. Befriend a mage!
The only reason to go holy while levelling is for the playstyle experience. That's actually a good argument, but pretty much the only one.