If I am riding solo, I ride with one ear but in( right ear- side away from cars). I hear everything just fine. It helps with motivation on work outs- different music helps push yourself farther or on an easy recovery ride something slower and fun(jack johnson station on pandora is great).

I would never ride with them on a group ride.

I feel it is a personal preference and people shouldn't be so negative to those who decide to wear them in one ear, as long as it isn't impeding you- it is ok.