Actually there isn't any trade-off since you only want one or two abilities from each spirit to begin with, the rest are just a courtesy since you don't have anything better to invest in, what you've done is allowed the player to spend the extra points to further upgrade skills that are already strong to begin with, likely making them insanely powerful if you continued the same progression. This would make my warrior very (trigger-)happy and my enemies very grumpy as they get blasted each turn with a high-powered rage ability.
Also passing variables to scripts isn't really a problem, if you couldn't do that, they would be pretty much useless now wouldn't they?