In most of the games I play in, we just sort of level up as the campaign progresses. I can buy that. My years of martial arts training tell me that a boot to the head is a very powerful lesson. Combat is a great teacher, assuming one survives. However, the theory always sort of fell apart for me when it came to wizards. Certainly, the wizard would gain a lot of knowledge about what worked and what didn’t after a few battles, but that doesn’t explain why a few weeks into his travels he suddenly knows how to cast a Fireball. Wizards always got screwed anyway.