toucanbuzz
No rule is inviolate
I think Dark Sun works better without Preservers. All magic defiles, period. Preservers were a cop-out so people could still play mages and not have to grapple with the consequences of defiling magic. They watered down the setting from "every mage hurts the world" to "only evil mages hurt the world".
It will really depend on players for this instance then. Maybe the best bet for defile magic is to give two options; one that shows defiling as a norm and defiling as a bonus. Let the group decide.
A major theme of Dark Sun is struggle and sacrifice, and its magic is a direct reflection. There is no such thing as something for nothing. And, to understand defiling is to understand its origin. When Rajaat invented magic, he "defiled" by drawing energy from life. His original experiments warped life, but didn't destroy it. See Windriders of the Jagged Cliffs.
When he spread knowledge of magic (with a very dark agenda in mind), he only taught "preserver" magic. Rajaat, by this time, had already discovered how to cast spells in such a way as to obliterate life and in turn gain great power. This "defiling" he kept closely to himself and his most loyal followers. Once he had found his "Champions," he stopped teaching preserver magic and turned to his Master Plan. See Defilers and Preservers. However, magic kept being taught, and by accident, untrained wizards found out how to defile. This eventually lead to the Cleansing Wars (Rajaat's champions vs. every other wizard), and it wasn't even close. And, the source book also details that since Rajaat, other sources of magic have been discovered: the Sun (how Rajaat made his Champions and dimmed the sun itself), the Black (you give more than you get), the Grey (links to the dead), and (later) the Cerulean Storm.
So, magic doesn't defile unless the wizard makes a conscious decision to take more than is needed. It doesn't mean "preservers" are always good. Those who defile might have good intentions and believe currently saving a child is more important than saving a tree (even though that tree will save more lives 10 years down the road, a neutral perspective in many ways).
My 5E DS conversion aims to capture this dilemma, this constant temptation. And, that's what defiling needs to be. That constant temptation. If it's a gimmick, it won't adequately explain the lure.