Bruce: When a wizard gets fireball, he can do a lot of damage in the round, but he only has so many fireballs. The fighter doesn't have that limitation. We have a lot of math and play evidence that tells us how long average parties or play is going to last, so we feel like we've got a good grasp of how to make the fighter and wizard relevant throughout the day.
Rob: As Monte mentioned earlier, some spells and options drop out and are replaced with higher ones, so that addresses some of the problem - you don't end up with all of those options. With that in mind, and the math backing it up, we can balance that figher damage to make sure that it stays relevant.
Monte: the play session that I envision with the fighter and wizard fighting together is that the figher is always better than the wizard. The fighter hits someone for 12 damage and then the wizard hits someone for 4, and the wizard wishes he was a fighter. Then that happens again on the second round, and the wizard feels the same way again. But then on the third round the wizard whips out his fireball and does 16 or 20 damage total and the fighter goes ahh, I wish I was a wizard. I want each class to shine and to have reasons to want to play that class.