But it started me thinking about what I'd do when I'm running future games. I don't generally think that multiclassing is a good bargain, anyway (it's not a bad bargain, exactly, but I think you're generally better off sticking with one class), and the XP hit just discourages multiclassing for no real benefit.
One benefit humans get is that they have a lot more flexibility in multiclassing without taking the XP hit, so if I were to scrap that rule, would I need to give them an extra racial feature to make up for it? What sort of thing would be balanced?
In my experience, very few characters are directly affected by this, but a more substantial number may be indirectly affected, as they avoid class combinations that would incur the penalty...