I don't even preach the "environment gospel" that some folks (most of whom feel more than they think) seem to enjoy belting out, mostly because it makes a lot of folks (like me) ignore everything that follows, so:
Early, pellet-type cats were a bad design, horrendously expensive (usually having to be obtained over the parts counter at the dealer$hip), and clogged up right readily. When they went bad, a lot of people just cut them out if they weren't needed for smog testing, because it could burn up the average guy's 2-week paycheck to replace them.
Current honeycomb-style cats are dramatically cheaper, flow great, and for anything but getting a few more hundredths of a second at the strip, won't cause any noticeable decrease in performance, will get rid of that "smokey" burnt smell that even a perfectly-tuned gasoline engine's exhaust will have without a cat, and some of them are really compact, so they fit in really tight spots. You can get a shorty for about 50 bucks.
In any case, if you live in a state that requires them on your year and model of vehicle, and you want to drive on the street, you have to have them, regardless (or else you can't get plates). Forget the rest of the world for a moment, and just think of yourself. It doesn't rob performance, and it makes the exhaust less noxious to you and the people that ride in your vehicle, so why not run them? You might as well. Just keep that engine in tune, so you don't go egg-man like some blow-dried disco-era reject in a rattle-trap Pinto.