[renowner]

I was just about to make a similar post. Monoclassing isn't worth it except in possibly Aquamancer, and only because it has a really strong passive that works well enough to make monoclassing potentially worth it.
Bonus skill slots is pretty irellivent, and not worth giving up another class.
Doubling the stats for using the class is a nice start, but it definately needs something else, too.

A cooler thing would be to make a 'super mainclass' only skill for every promoted class, that's only usable if your main and sub class are the same.