The new Defiling Magic system presents two reasons to defile: A boost in the spell's power and to have more spells/day.
A particular area is rated for the maximum spell level of energy that can be drawn from it, and the radius around the caster of land defiled by drawing that energy. More fertile terrains with more plant life have a higher spell-level cap, and cause less destruction when used for power. The soil is turned into barren lifeess dirt, and any plant life in it withers into ash. It takes a minimum of 100 years for the terrain to recoup it's vitality.
When you defile an area for power, you can choose to draw more energy than is actually required for the spell, but no more than the terrain's level cap or your maximum arcane spell level (whichever is lower). The excess energy can be used to add effective caster levels to the spell on a 1-for-1 basis, or to add a bonus to the Spellcraft check for adding a Metamagic feat (another new system in the file). Regualr arcane magic users that use Defiling Magic only have to draw the difference in spell energy from the land, allowing them to add +3 spell levels of effect to a spell of any level, so long as the terrain can support up to 3rd-level defiling (for instance).
There is a problem with this power however: addiction. Whenever an arcane spellcaster uses Defiling Magic they have to make a Wills ave whose DC is based on the TOTAL spell level. If they fail then they become "Defilers": spellcasters addicted to the rush of power from Defiling Magic.
Defilers must power their spells ENTIRELY from the use of Defiling Magic, they cannot recharge their spell slots fully by resting like other arcane magic users can. Resting just allows them to sketchilly recharge thier spell slots, but the actual POWER for their spells must come from Defiling Magic. This inability to fully empower thier spell slots has a side effect: Defilers have mroe "room" for extra spell slots. Defilers get a pool of spell levels that they can use to cast spells of any level they can cast normally, but only once their regular spell slots of that level are used up.
A character can "kick the habit" by spending days in meditation as they slice-n-dice the adicted portions of their soul away (spellcasting class levels). At the end of this time they must make an ability score check or die permanently! If they succeed the check, they are no longer addicted and can even recover some of their lost character levels if they succeeded particualrly well.
And that in a nutshell is it. Only the numbers have been left out.