> What about this:
> Humans, being the short-sighted "I want it all, right now" bastards that
> they are, can be multi-classed but NOT dual-classed. Dandelion-eaters,
> elves, can dual class, as they are IMO more prone to the "Oh, I've been a
> warrior for a hundred years...It's getting boring. I think I'll become a
> sculptor or a mage..." line of thought.
> Yes, I know it will play havoc with your campaigns.
I think that's a fine explanation for a "house rule" reasoning why it's
plausable for Dual-Classing to be the domain of Elves and Multi for humans.
I think I personally will (for now) stick with the rules as written, with
this excuse (or something like it):

"Elves, being the semi-divine creatures that they are, are able to think on
multiple levels at once, and grasp vastly different concepts, so they can
multi-class. Humans, being bairly above the ameoba level, can bairly walk
and chew gum at the same time, so they can only understand one class at a

(and there, I said something else in favor of elves).