I... had not considered a -feat- for it...Defiling should be better than preserving, full stop. That's why there's the temptation to do it.
Defiling should be the default for arcane casting, and not defiling should take either an at-cast time penalty or require investment of character resources (a preserving subclass or a feat).
A Preserving feat that allows you to cast your preserving spells without a cast-time penalty would work well in the "Preserving at a Cost" variant option.
It's also got mechanical problems. Like destroying the precious, precious, water that you need to survive.@TwoSix, @Steampunkette
I realize defiling needs to be "tempting".
But trading mechanical benefits for narrative punishments has been causing D&D to become broken, since the days of 1e.
And if a person were to use the Journey system, as I suggested, you could have it literally destroy the Spell's Level in Supply. Which could cost you several days worth of survival.