To sum the main points:

* Reducing the numbers of abilities to give more identity to the classes: understandable
* Limiting cross-class abilities: understandable
* Fixing to 15 abilities and 11 traits: problem

It's not the number of abilities per se that is the problem, rather a question of how rigid is the system. Some classes may be effective with 10 abilities, others with 17 (made up numbers): this affects magic users more because traditionally they had a larger pool of spells to pick from (even if you take into account the scaling).
If the "15/11" rule was just a rule of thumb (15+-STD, more or less), I doubt there'd be any problem. If it were 15, *period* it would be due to intrinsic differences between the classes.

And to reiterate, this doesn't touch anything on "useful" or "useless" abilities, it's an issue lower in the stack.