I'd like to see this implemented.
Or even a % for misfire when reloading the crossbow (sweaty fingers)
Why?
It'd be funny, and honestly, I'd go for the realism argument here. I bet it's happened irl.
So if you have low wpf, the chances of shooting yourself in the foot are higher, and you can manually shoot yourself in the foot. So aiming down castle walls will add some apprehension.
Honest to god, I think I'd use an xbow if this was implemented for real.