Kamikaze,
I disagree. Defiling is specifically described in the setting as being more powerful. In 2nd ed, without all the spell affecting feats of 3rd ed, they let defilers gain levels faster than normal.
There IS a mechanical disadvatage: defiling itself.
In 3rd ed I'd give them the effect of metamagic feats as bonuses.
I agree with Kathan's idea and my system would be this:
When a spell is cast as a defiling spell, the user can choose a free metamagic feat to affect the spell (limited by his level, a table with level and available powers is needed).
When the spell goes off, a defiling effect burst goes off, affecting ALL creatures, except the defiler (and familiar) in 5' radius per spell level. (a feat could allow specififc allies to be unhurt...but the defiling does require life energy or the spell fizzles)
Damage is 1hp per spell level AND caster level. Will save is allowed for no damage, DC is 10+ Int bonus + spell level.
A spell cannot be cast as a "defiler" spell without the presence of significant amount of living organisms. Thus it cannot be used in most deserts, unless there are living victims nearby to sustain it... So most defilers would keep slaves as "fuel"...muhaha
There are several disdavantages to using defiling magic:
1) Depletes life, thus, only trees and animals could survive more than one casting in same area, and even they will die with several castings. Eventually reduces areas to lifeless deserts, RP, but important.
2) Dust, since things crumble to lifeless dust, the defiler is swathed in dust (mechanics of that's debatable).
3) Most creatures kill defilers on sight, for several reasons, Again RP, but it should make life brutally nasty for PC defilers, as no one will care for them. Only the most corrupt folk will have anything to do with them.
Folk need to stop thinking in metagame ways...Dark Sun is not a "game balanced world". Defiling is more powerful than normal magic. Tough! The defilers (Sorceror Kings) wiped the preservers out, almost. Hard proof, in the game setting, that defiling is indeed, more powerful.
That is not a bad thing! It's good, it's different.
PCs who are defilers should be harrased to heck, it goes beyond "tree hugging eco hippies" as some may see it

In a lot of primitive cultures, nothing, absolutely nothing, not even the death of family, is as catastrophic as destruction of your wells and fertile land. You can have more kids, but without food and water...they have no chance, ergo, folk who poisoned wells etc suffered the most horrible punishments available.
So defilers are literlaly, Public Enemy #1.
And a mob of enraged towns folk has great grapple abilities and thus, defilers get slaughtered
