Single speed road if not for commuting or stuffing around is not nearly as fun as geared road. SS mtb is better because it presents more challenges with climbs/traction/obstacles etc. While fixed road has its place in training, ss road has no bling, still freewheels so hasn't got the training benefits of fixed, wants to be fixed and geared at the same time and you can't race on a SS. And running a gear high enough to sprint in for a whole, otherwise slow, ride is no fun.