In my opinion this is a great Idea.
Gives more motivation to do train INT.
Don`t forget that this is not the final version of PS.
The mental stats should determine the power of the spells, but it doesn`t work yet.
At the moment it doesn`t matter if your char is stupid or genius, your spell causes the same effect.
When magic is fully developed it will make a difference.
Imagine you try to cast a spell in a battle, the opponent almost standing at your feet.
When you have less will power you probably would become nervous and begin to stutter, or worst: low intelligence will let you forget the right spell

Your spell would fail and your lack of mental power would be a disadvantage.
For me the amount of mana points is reason enough to train stats.
I have a Kran char with -1 mana points

...i decided to let him be just stupid, he can`t even speak in complete sentences...
Greetings
Herleva, Freelancer