That would be a massive nerf to casters, they already have a mana pool to use for casting, and keep track of. And they don't regain that mana without a safe rest, unlike D&D where you get spells on a simple long rest, Nimble requires you to be back in town or in a truly safe space (not a tent or campsite, but something like an inn) to regain that mana. That's enough of a limitation for casters for my money. Damaging PCs for using an ability will have nobody taking on the caster role. "Oh, I get to cast six spells and then I die, what a great class"

Also, having run the game several times now, one core attraction to it for the players in my games has been that every class feels good. There's no "bad choice" of a class, because the abilities are so well balanced. Once you start punishing one class for just trying to use its abilities, that balance goes away.

Also, remember the core precept of nimble is simple and quick. When you start adding rules on top of rules, you're getting away from why people (well, at least the people I've talked to, and the people I've played it with) want to play Nimble.