The bicep has 2 functions, to raise the lower arm & to supinate the wrist. So rotating during db curls works them completely. I feel like db curls with my wrists supinated (not rotating) the whole time work better for some reason though. I switch off.
The only advantage to rotation during bench or shoulder press I see is recruiting the rotator cuff. I think the reason you see so many guys doing db bench press with a neutral or rotating grip, is that they think they are doing flyes. They want to use much more weight than they could use for real flyes, etc.