Right, and that's primarily because the leverage against its drag works in the rider's favor. A dynamo is going to get its power one way or another, and dragging against the tire -- at higher RPM within the dynamo, mind you -- slows down a wheel more than when it's in the center of the hub.

Plus, a dynohub never slips in wet or icy weather like a tire-driven dynamo can (if it's out of adjustment, anyway).