I didn't say use shadow form, just shadow spec. Shadows priests are good for soloing. I've got one myself.
Edit:
well I had one, I got banned.
I was explaining why they werent good for soloing in shadow form. Now even out of shadow form, they are one of the worst soloers. They do awesome damage if you enjoy burning your mana like hell. But, that then takes time to drink up. So your xp/hr goes down. Name a class that is worse at killing effectively and fast.