theoretically you are right, the voltage (from + output of 4 diode bridge to - output of diode bridge) can be negative some volts only (that is the voltage drop across two diodes). It can only do that during the zero crossing of the AC input voltage. So considering mains voltage rectification, it can't go negative (from a practical standpoint of view).
The only reason that I can think of, is to avoid reverse recovery issues with the 4 bridge diodes. If at a zero crossing all 4 diodes conduct due to the inductor current, you may get a voltage/current spike during diode turn on. By adding a seperate freewheel diode, all 4 bridge diodes will not be on at the same time.