IMO, this removes one of the main drawbacks of being a defiler: the lack of subtlety. A defiler should be more powerful magically than an equal-level preserver, but it's super obvious when they cast spells because there's a great big black circle around them.
Ideally, defiling should be an option for preservers who need an extra boost, but if you do it enough you become a defiler. It's a bit like the Force in that way – the Dark Side is happy to help out a Light Side user in a tight spot, but it exacts a toll for doing so, and if you do it enough the Light Side will stop responding.
This was a popular AD&D design: evil is the quicker path to power. We saw it with Dragonlance's Wizards: white robes (good), red robes (neutral), or black robes (evil), in which black robes advanced in levels quicker than their counterparts. But, it never seemed to make sense. Why would my choice to learn Abjuration or Necromancy be dependent on my world views? Wouldn't it instead be my application of those powers? Could an evil wizard not use abjuration to deflect damage away from himself as he burns down the orphanage?
Defiling fit the same bill. It's more like a drug than anything else, and it doesn't make sense that a person would be committed to a subclass or AD&D class solely based on one, or a handful, of defiling choices. Even Preservers can engage in non-good acts if they're a hardcore believer that "defile once" and you deserve to die regardless of the reason. And the reason is the theme on Athas. Why not, just this once, suck one of those trees dry to stop the Templars from arresting or killing civilians who did nothing more than turn a blind eye when you ran through their shop to escape? And after that one time, why not, just that one more time, suck those faro crops dry to make sure the bandits don't raid that farm's cistern? And after that next time, you keep making excuses. It doesn't mean you're beholden to defile every time. But the bar that was once perhaps set so high for what would cause you to violate precious life gets a little bit lower, and that's the real toll.
But, one who defiles has never been required to destroy life to cast a spell. It was taught as a secretive weapon, an advantage. Indeed, sorcerer kings keep defilers on reserve to serve them while destroying all others in their cities. They aren't going to keep anyone around who would suck their precious gardens, parks, and crops dry when doing routine casting. The arenas keep necromancers around to animate the dead for entertainment. They can do their job without taking more than they need, even if they'd defile in a heartbeat to save their own necks.