I wonder if it could be abused for more than just a "different way to multiclass". For example, I think something similar to racial classes could be added, for what 3E had as "High LA" races. Basically, you don't get the full features of your class for getting more racial features, to play something exotic, like a Mind Flayer or even a fully fledged Dragon.
I have never created a 4.0E multi-class character, nor are there in any of the games I've played in.
How does multi-classing work, and how will it be different from a hybrid-class?