I understand it a little differently.
The way I understand it is that wild mages memorize spells just like any other mage. However, they have chances that their spells will go wacko on them periodically. They also have a chance of their spells being super-sized. For instance...a wild mage who casts fireball may: 1) Cast a normal fireball 2) Cast a triple or quadruple strength fireball...like 30d6 or something 3) Try to cast a fireball and have it fizzle or maybe turn him into a squirrel.
Now *that* sounds like fun!
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Now where did I leave that doughnut?!