I haven't seen anything that says the Shadow is specifically "meant to hurt". Just because it does not heal doesn't mean it only has harmful applications. The Shadow isn't really any more specific than Fire or Ice arcane magic. Fire can be used to burn a man, ice to freeze him, but both also have practical, very peaceful uses. We typically only see the martial ones due to the nature of the gameplay. Shadow deals heavily with death, yes, but that can simply being a sort of spirit guide for those who linger in death or using magic to hide allies and innocents from their enemies.But let's just say that he doesn't follow the teachings of the Cult. Why would he seek the Shadow? No doubt like other Draenei he was a devote follower of the Light? The Shadow doesn't heal. It is a very specific in the fact that it is a type of magic meant to hurt, but it isn't strong against their (Draenei) main foe, the Legion. You would really need to define his goal for taking up the Shadow for this conversation to go further.
This gets into a whole new argument. It's -very- logical for a race devoted to stopping the Legion to take up the Fel, or a magic close to it. How can they expect to truly defeat the Legion without an intimate understanding of what they're really going up against? And what better way for a race such as Orcs, that have been enslaved by the Legion, to get back at them than to enslave the demons themselves?Logical for a member of a race so devoted to stopping the Legion to take up a magic so close to Fel? No. I could see you making an argument for learning about it, but actually using it? Not a chance. Like I said you really would need to define what his purpose for taking up the Shadow would be.
But the Shadow doesn't seem to be any closer to Fel than, say, Fire arcane magic is. To more directly answer your question, the necessary balance between the Light and Shadow has been stressed all throughout. Is it really that difficult to picture someone who tries to straddle that line, to embody that balance? Someone whose devotion to the Light calls them to understand it in all of it's facets, even when there is a void? Yes, a Naaru falling totally into the void is a "catastrophic loss", but that's not what we're talking about here. That's more akin to a Mage joining the Shadow Council. It's a possible result, but not the only one.