With no fluid in the PS system, at a minimum you will mess up the PS pump. Then you will also have to deal with the danger of not having control of the steering system due to lack of "assist" from the hydraulics (on non-thereof) and the fact that steering will be "loose" but "heavy" due to the lack of fluid. The universal joint (coupling) will also be subject to more strain as it will not move as easily.
If you look at the diagram in this thread, you will see at least 4 places on the steering column where joints will be at an increased strain (stress) due to lack of power assist.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=1250417&fpart=1&PHPSESSID= 
