Had my exhaust replaced today, and discovered that my cat. converter was hollowed out by the previous owner.  Funny thing was that I passed emissions (Colorado) with flying colors, just a few months ago.  I've got a 93 16V sidekick with a newer (96+) motor in it.  So you might be able to get by without it in some cases.  I guess that means these motors do a pretty good job minimizing harmful emissions on their own.  Or the emissions testing is jacked.