I think you forgot the "L" for the inductance.
The equation is Vemf = L dI/dt.
The inductance can be 1 mH or so, but the switching times will be more like 1 µs, so you still get silly peak voltages if you don't have diodes.
The Schottky diodes have a lower voltage drop and faster switching times, which is why they are preferable.
If the circuit is controlling the current by PWM, the supply will only be interrupted for very short times. The current will continue to flow in the windings, through the Schottky diode, until the power is restored. See figure 4.6 of
Jones on Stepping Motor Current Limiting. I know that is for stepper motors, but the current control by PWM applies to any inductive load.