Per my post below, sort of cool timing. From Cycling News:
Does training on a single speed bring any particular advantages? Obviously riding a single speed leads to pushing harder at times and spinning faster at other times, but does this bring any benefits to the rider when he transfers back to his geared bike?
Scott Saifer replies:
The single speed does force you to push hard on the pedals, developing power which you can use on your geared bike, and force you to spin higher cadences, which can help you develop your spin for sprinting or routine riding depending how bad you spin is to start with. If you went out and deliberately trained the same variety of cadences on a geared bike, you'd get the same benefit.
The major benefit of the single-speed is that it is different and so can keep training fresh and interesting for a few more hours now and then. I would not suggest using the single speed exclusively. If it is a fixed gear as well as a single speed, avoid spinning out on down hills. The single speed does not help make you smoother when the pedals are driving your feet, but only when you feet are driving the pedals.