It's not always just about not having enough time. Sometime people just don't want to level a certain class because it isn't fun. Noone wants to play something that isn't fun. That defeats the purpose of playing the game in the first place. Anyhow it's all about balance.

If a class can't at least be self-sufficient to the point where it's viable to play, then it's SE's failure at balance. I agree that levelling other classes for abilities should make you stronger, but building a class that can't sustain itself if so desired is just stupid.