Hmm, is it really correct that you do not need any current limiting when using steppers with a L293D? I tried to use a similar setup as dutchboy, a power supply with 12V and 1.5A. But the L293D become verry hot after seconds and the stepper doesn't work correctly after some seconds. So I reduced the input current to 600 mA and now it works. But perhaps it is only necessary to use a heatsink.
The L293D will go into thermal shutdown if it gets too hot. So you need to heatsink it adequately, especially if you are running it near its maximum of 600mA per winding. This is difficult because it doesn't have a tab you can bolt a heatsink to. You really need to use a PCB and solder the ground pins into a large ground plane on top of the board.
The main reason to use current limiting with stepper motors is when you are driving them at higher than normal voltage to overcome the inductance of the coils and get the stepper to move faster. In your situation, reducing the supply voltage would have been sufficient to reduce the current, and therefore reduce the heat dissipated in the L293D.