The ladder test only lets you assess vertical location/movement between charges. The OCW, by resulting in a 3-shot group (or you can do 5 shots), allows both vertical and horizontal movement between charges to be evaluated.
Also, since you're firing round-robin style, OCW mitigates the effects of fouling build-up over the course of the string.
Shooting more than just 1 shot per charge also mitigates against 'flyers.'
It's just an all-around better method to evaluate loads.
