The ecu's use VSS for different things in the cars. For example, in the SN95 cars, the ecu turns off the electric cooling fan when the speed exceeds 25mph. VSS signal is used to limit top speed in cars that have that feature. In the fox bodies, one thing the VSS signal is used for is to hold the idle speed a bit higher when you're decelerating. If you have a manual tranny, slip it into neutral and watch the tach. It'll hold it around 900-1000 rpm until you roll to a stop - then it will drop down.
My car has never had a VSS hooked up -- 10 years, 43k miles. Doesn't seem to have posed a problem.