Besides the reasons others have mentioned, there is a SERIOUS SAFETY ISSUE with using a variable DC drive as a poiwer supply. Most of these drives do not isolate the output from the AC line as a true P/S would. To make matters worse, in many variable DC drives the common return line for the 0-10 volt speed reference signal (used for the speed control pot) is actualy floating at -60 volts!
Very good points. A neutral should never be grounded outside of the main panel, and it likely would be if the controller didn't use some type of transformer.
I built a small PWM motor controller for use in controlling a small Dremel-like 12V motor. It worked well. However, I added a capacitor and low ohmic value resistor to the output to smooth out the square wave pulses into at least strings of ramped pulses. I don't like what Lenz's law taught me with respect to switching inductances on and off. Even with a reasonable back-emf from the motor, transients can wipe semiconductors quite handily. Although, the power MOSFET I used had a back-biased diode internally for protection.