Without the motor flywheel diode, not much current will flow through the motor coil windings, thus no substantial magnetic field will be created. the power gets wasted creating high voltage back EMF. the mosfet's body diode has no effect. the spikes of back emf also affects the gate signal via the junction capacitance.
the result is erratic control of the motor, mosfet high on resistance, high voltage spikes, all sorts of nasty surprises
