2000 Nissan Sentra Should I change my catalytic converter?

Computer problem
2000 Nissan Sentra Two Wheel Drive Automatic 121000 miles

I had my check engine light come on a while back. I took it to a mechanic and he checked it on the machine and told me that my catalytic converter is bad. Because I was not to excited about the price of the part, he told me it was not necessary to fix it. The only thing it is doing is hurting my gas mileage. I travel a lot and want to make sure its not hurting anything else. Is there any possibility he is wrong and that it is hurting my engine? Should I go get it fix?