So in the case of fighting Skeletons and Orcs, a Warrior would be purchasing Mace Expertise and Axe Expertise to complement his strength. A Wizard would use Lighning Bolt and Meteor Strike which draw upon his vast mana supply. And a Thief would use Hide in Shadows and Lock Pick to circum-navigate his enemies and get the goodies (for which he should receive fair experience, for avoiding rather than killing the enemies).
How do you want to play?