There’s some aero benefit to getting rid of the seat tube although there is now nothing to shield the rear wheel. Plus the boundary layer of air moving forward off the rotating rear wheel creates some aero ‘churn’. However the main reason why beam bikes (see Softride) have been popular for triathletes is the suspension effect of the seat beam saving the back and legs of the athlete for the run…friends who have done several full length IM’s say its all about saving it for the last five miles of the marathon leg.