Critics dismiss the application of a fixed-gear trainer to improvement of road bike technique as analogous to practicing on an ostrich as preparation for riding a racehorse. Ideological opponents of the fixie advocate a purist approach. To get good at riding a road bike, ride it. Controversy aside, a fixed-gear trainer offers potentially useful challenges. A fixie is harder to control than a road bike.
More finesse is required.
Persistently turning crankarms force you to execute all maneuvers while pedaling. It's not just for knuckleheads anymore. And the emergency separation of tires and pavement is complicated when launch occurs with cranks anywhere other than parallel to the ground . There's greater consensus on the value of fixed-gear road training for physical condition. You have a limited range of capabilities.