For races I think is a rule that you must have one but it wasn't being enforced 100%. Besides for races you never know if someone front light failed and they are standing on the trail. At fast speeds you will want to see some sort of warning.
At the Tuesday night SWAMP rides, I seen peeps with very small red blinkers or a small blinking light in the jersey pocket. But since we ride in packs you really don't need it. It is more distracting then anything when we all ride bunched up like that. that is why some ask to turn if off.
When we ride the Dung Bettle ride, uban assalt, or the christmas lights ride, we encourage everyone to have one to be seem by drivers.