Yes, but with one very important caveat. I want them to scale with character level, not caster level. As I mentioned in the multiclassing poll, I think this is about the only way to get multiclassing to work in all cases. Otherwise, the system rewards specialization too much and multiclassing fails.
So, if you have three guys: Wizard 10, Fighter 5/Wizard 5, and Fighter 9/Wizard 1, and they all have Fireball (assume they took their wizard level late enough to do so), then all three of them would deal 10d6 damage with their Fireball. There is no reason why Fighter 9/Wizard 1 should only deal 1d6 (or even be unable to cast it), or why he should have to spend a feat to get it up to a measly 5d6 damage. Not being level-appropriate means your abilities aren't level appropriate. Period. No sane player would waste an action on that unless the DM through 50 kobolds at the group at 10th level.