I'd say the rule is limiting, unnessecarily so. There are many concepts that can be done effectively with a Paladin or Monk (esp. monk) multiclass that almost can't be done, if you don't allow the multiclass by defualt.
I mean, I can understand the flavor justification for it (both classes are supposed to represent focus), but IMHO the benefits of eradicating that restriction would be MUCH, MUCH outweighing the benefits of keeping it (which kinda boils down to 'I'm focused!,' which you can do without the restriction).