Mixing skills in this game is an awesome feature, also covering a point that a character can direct the skills to the way they want.
I agree, but all your skills will look exactly the same. The only unique skills are ones restricted by class.

If by restricting them, they could make them more unique looking and interesting, that would be cool. I like the idea Cairdeas came up with as well. Anything that brings more diversity to a particular weapon is a good idea to me.
And that is essentially the point, balancing isn't really important as it would be done after the fact, each weapon skill would be different looking, and, therefore, each class would be more unique.