Well, if Charisma allows you to persuade people, it could also work the other way round. Meaning that it prevents you from being influenced, as your personality is more prevailing. So if you have a weak "personality", using Force powers "influences" you and tires you (= more FP spent), while with a strong one you are not dominated by the Force. I guess that joins Kreia's vision of the Force.
In Soviet Russia, Force Powers Uses YOU!! :P
"Do you think I seek the death of all living things? There is no victory in such things.
I do not want to win our war like this, little Jedi. When I win, I wish it to be because I was right, my teachings true."
- Kreia, Jedi Master, Sith Lord