Celebrim
Legend
"And they're a mistake. I'd contend that you put the world-story through bigger contortions explaining someone who multiclasses into barbarian than you do explaining a monk who multiclasses into something else, then takes more levels of monk."
This is an example of misidentifying the problem.
The paladin and monk multiclassing limitations are fine as optional rules (of course I tend to see all rules as optional). They are mistake only to the extent that the rules shouldn't tell you how to play. If you want to have paladin and monk multi-classing limits, then that has a lot of flavor. Nothing wrong with that. If you want open multi-classing, well, it's probably not going to be broken (although both classes were heavily front ended) and that's a valid flavor choice as well.
But the problem here is that monk, barbarian, and paladin all try to tell us too much about the class. Must you really be a primitive member of tribal warrior society to be a 'barbarian'? Is 'barbarian' really the best name for the concept here, or is the term just confusing the matter. Must you really be lawful good in order to be a divine champion? Do all skilled hunters eventually acquire divine magical abilities? Is a class loosely based on a Celtic animist priest really culturally appropriate for every setting?
Doesn't that tell you how to play in ways that are at least as restrictive as the paladin and monk multiclassing limitations?