I think another problem with muticlassing in 3.x is that there's little synergy between classes if you do it, unless it's just a one level dip. A Fighter 5/Wizard 5 is a worse warrior than Fighter 10 and a worse spellcaster than Wizard 10, and his two skillsets don't work well together - there are only so many actions per round, so he can't do both at the same time. If he's more powerful than Fighter 10, it's because of the gross class imbalance. A multiclass character ends up doing two things, but can do neither well enough. WotC seems to have realized it at some point, and we got the martial manuever progression and various feats that let you advance class features without taking levels in that class. It's an open question whether or not they'll implement something like that in D&D Next.