Speaking as someone who is deeply allergic to the idea of playing a single class toon --- I still think that single class should be encouraged (and rewarded) by the game design, and that the more exploitative flavour of the day MC builds should be actively discouraged, whereas people's deeper multiclassing, more respectful of the spirit of the game, should not be hindered in any way.
Multiclassing needs to remain kind of an art form, instead of being a method to subvert the deeper principles of the game.