several things:

2 o2 sensors
with a test pipe the 2nd o2 sensor is going to trip since it cant detect anything diff than the first...meaning the cat has failed to the ecu.
i dont know about you but i personally am sick of the check engine light being on all the time (i have an egr problem)

changing it out is a pain in the ass unless you have a lift. the bolts tend to rust quite quickly and between rust, dust, heat and humidity its pretty much a hassle to do no matter what....but doing it while laying on the ground is worse...