Considering that Larian are still using the disposition slider for merchants, they could set Friends up so that when you use it, you get the disposition benefit, and thus price discounts etc., but when it wears off, they take a negative hit by the same amount - so next time you speak to them, they'll like you less than they did before the charm, becuase they're annoyed at you for using magic on them.
This might be stupid question ...
But what stops you using the same spell again? O_o
