The thing that never made any sense to me about dual-classing was the kit restrictions. They're the wrong way around.
Look at the way our education works: you start out general, and then specialize (even if it's in something else). In BG2, though, you start out specialized, then realize (oh sh*t! i don't wanna be an assassin!), at which point you become some kind of fighter-bum.
It would make more sense if it worked this way: dual-classers would have to start out as their true classes, and then the second class would almost certainly have to be a kit.