The prop blast moving over the carb opening can create a vacuum effect (venturi). Using a velocity stack will decrease this effect. Seeing fuel dribbling out of the carb is a sign that raw fuel is being siphoned out of the carb by the venturi effect. The effect of this is obvious.
However, sometimes this doesn't happen. I have one 26 cc gasser with a V-stack, and my G23 runs perfect without one.
BTW, you will usuall get the best effect from the V-stack with the opening angled back, away from the prop (at least to some degree, if your v-stack has an angled opening).