Nigel: He is using the regulator to feed the motor because there is no current limiting setup.
uaefame: It doesn't sound right, but if you limit the current instead, you have either a PWM or chopper based system. This is the next step as it provides more torque and hence a greater top speed. It also puts you in a position to do microstepping.
I have a large 2V stepper motor which I run from 10V. When I get a proper power supply setup, I will be running it from 24V. It is current limited to ensure that it doesn't overheat.
There is a limit to the max voltage, because of "iron losses" from the high switching speed of the current limiter.
The highest advisable supply voltage is 32 * sqrt( Inductance ), where Inductance is in mH. Another method (Less accurate) is to run the motor upto the rated voltage * 10.