VSS question

[Andrew Choset <agc3@cornell.edu>]

In a manual transmission car with a cable driven speedo, and no cruise, is
there any real purpose for the VSS?

Background to my question--

88 IROC-Z, L98, originally 700R4 with cable speedo.  Swapped in T56,
replaced speedo with AutoMeter electric speedo.  Stock speedo incorporated
an IR pulse generator to feed a signal to the VSS buffer box.  As it sits
now, there is no pulse generator, and the VSS buffer box gets no signal,
therefore the ECM gets no VSS signal.  As a result, code 24 would set.  OK,
use a jumper to close the Park/Neutral switch, to fool the ECM into
believing that the 700R4 that no longer existed was always in park, and
therefore the ECM didn't look for any VSS input. Did the trick, no more
code 24.  

However, it soon became apparent that I've got an idle issue... upon coming
to a stop (trans in neutral), the idle speed will often hang out around
1500rpm, sometimes even a little higher.  Is there a possibility that this
is a result of not having a VSS input?  The problem does *not* occur
always, in fact, sometimes I'll go a week without having the problem.  

Any ideas?

Thanks in advance.

Andrew Choset