[Rob2197]

Marketing and hype. The grooves on a driver are primarily cosmetic and tradition. Angle of attack at impact, mass distribution of the head, and shaft tip dynamics influence spin off the driver (other things as well I am sure). In fact most of the long drive guys use drivers with no grooves. It allows them to get thinner faces without caving the faces in.

I agree totally. The grooves on a driver are so thin that they can't make much difference. However, spin is needed to get any type of trajectory it's just that this spin is created differently by drivers than by irons. Drivers have bulge and roll influencing the direction of the compression on a ball, irons do not so they need grooves. In fact, an iron with debris filling the grooves significantly affects spin and distance.