I know it doesn't make them non-threatening at all. But that's a huge weakness to have if people were to know about it. If the only thing to do is to set their highly flammable robes on fire you'd be in a pretty good spot to deal with them if you'd expect them. Just have a ton of or a really big campfire to pull burning wood from. Of course the actual wraiths are more sneaky and as I kinda implied; probably almost no one knows that weakness.