Typical motor PWM frequencies are certainly less demanding than a fast switching converter, but both reverse-recovery and switch-on time are important. For a low-power (and/or low inductance) motor and slow transistors, a plain diode is probably OK but since Schottky's aren't exactly hard to come by, it's best to use them for this purpose.
Considering the case of an H-bridge with 4 freewheel diodes:
- if the reverse-recovery time is long, there is some shoot-through via the conducting diode and its opposite transistor (on the same leg of the load), which can cause the diode or transistor to fail,
- if the switch-on time is long, you get high-voltage spikes appearing on your transistors, which can kill them.