If you want some numbers, let's assume the voltage-spike generated at switch-off is 1kV and the motor winding resistance is 10Ω (any of you guys think those figures unreasonable?). With only 100Ω in series with the motor the current through the PIC internal protection diode (let's say having 100Ω resistance too) could be ~ 5A in the absence of external protection diodes. AFAIK PIC protection diodes are rated for only a few mA, so the diode is likely to fail. With external protection diodes rated for 5A you might get away with it, providing they have a lower forward voltage drop than the PIC internal diode and hence pass the majority of the current. I wouldn't bank on it.