The ideal, of course, is that if offered either a straight fighter, a straight wizard, or a multiclassed fighter/wizard, the decision on which you play shouldn't be predicated on "the other two suck." The wizard, devoted to magic and eschewing martial skills, should be a better wizard than the fighter/wizard. Right? Otherwise, why play a straight wizard?
I assume that folks don't really want the fighter/wizard to be just as good at casting spells as the straight wizard, but (martial talents included) capable enough at wizardry that he does not regret dividing his career.
Cheers,
Cam