One common theory in WoW is that while Light magics are used selflessly, Shadow magics are powered by (and feed upon) one's selfishness. Thus, most RPers feel it is more or less impossible to be a practitioner of both.

Of course, this isn't to say a Shadow Priest cannot do "good" acts, it's just that it's extremely difficult to wield the Shadow effectively when that good doesn't directly benefit you. There has to be some degree of selfishness involved.

One RPing guide for Priests in WoW says:
"If you are a Holy priest, you are loyal and selfless, always willing to risk your life for the good of others. However, if you are a shadow priest, then you are the exact opposite. Greedy, selfish, and ignorant. Will do anything to get power. Exploit it, use it for your own good, let others suffer if it will grant you power. Much like a warlock, you are not trusted, even hated by many."

Sounds a lot like Conjurers and Thaumaturges, huh?