I actually really like this system as it gives you a good understanding of how other classes operate. It's particularly helpful if you're a tank, for example, because you can learn the strengths and weaknesses of the other tank, what kind of healing to expect from your healer, the best way to position mobs for your melee party members and so on. True, you can look this all up and have an intellectual understanding, but you can make snap decisions better with actual experience.
That said, it is pretty rigidly set up and it'd be nice if there were more room for weird stuff, like, say, crossing Blizzard 2 to a tank class so you can bind enemies. Or crossing traits as well as skills so you can be a Warrior with a million HP. But I guess that would break the game just a little bit.