Augment Defence is good for any class, but mostly Mage I think, because once the enemy gets too close, then they'll need all the defence they can get. Spells won't help them there anymore. (Except for that shield spell perhaps...)
Tovi, If you're REALLY good as a mage, most monsters won't GET close to you <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
