IMO, the differene is that a taper crimp is used on auto pistol rounds that headspace on the case mouth. If you roll-crimped this kind of ammo, you would mess up the headspacing arrangement.

Roll crimping is for heavy recoilling revolver rounds in which you ABSOLUTELY MUST prevent bullets from creeping forward out of the cases under recoil... bullets that crep forward could end up sticking out of the front face of the cylinder, preventing the cylinder from rotating AND preventing you from oppening it to remove the bad rounds........