I've had two friends "ejected from the saddle" and sustaining serious injuries. They both said, "You spend your whole like perfecting your reactions on a freewheel bike. On a fixie, those reactions are dangerous." OTOH, I like riding SS with a freewheel. You can still pedal downhill, you don't have the problem of the pedals driving your legs around, which leads to knee problems for the inexperienced, and you don't get injured. OTOH, there's no training effect which translates to improved performance on a geared bike.