Neat and simple. Doesn’t it however give too much power to spellcasters, especially those with lucky rolls? And understandably, defiling can be dangerous while within a city, but who cares when you’re travelling in the middle of nowhere, or exploring ancient ruins?
I have a group that spends a significant time outdoors, none of them plays any good characters, and most of the random combat encounters occur during their travels (it’s one harsh setting, we all know), so it comes naturally for one to defile and empower their magic while the others turn a blind eye. It just so happens that the players are excellent RPers and they play their part of despising defiling magic, so defilers in the group (so far, only one) go to great lengths to hide their abilities, for fear of being exposed and subsequently, hunted down.