Paul Farquhar
Legend
Consider why Athas has defilers. it's because the world almost completely run out of natural magic. All living things require at least a little magic to be alive, defilers fuel their spells by draining that trace magic from the living thing around them. Thus, if a defiler where to travel to a plane with abundant background magical energy, they wouldn't defile. A wizard from another plane who landed up on Athas would have their spells fail due to insufficient magical energy, unless they learned to defile.Not to side track(too much), but I've always been curious about defilers leaving Darksun to other realms, and wizards traveling to Darksun.
Of course, one of the reasons Athas is drained of magical energy is its isolation from other planes. If you could open up a stable portal to another plane magical energy could be drawn from there (see Doctor Who: Logopolis).