They're not necessary for proper engine operation, but you won't find a down pipe that will bolt to the manifolds without the pre-cats.
Are you trying to make the TJ computer run in a CJ, or just swapping the motor over? The TJ is OBD-II, which makes a difference. While it will run without the catalytic converter, the computer will throw a code if it "sees" a non-functioning(or missing) catalytic converter. It has "upstream" and "downstream" O-2 sensors, they up-stream sensors to check A/F ration by exhaust gas analysis, and the down-stream set to confirm the catalytic converter is functioning properly. Remove the upstream sensors, the engine runs open-loop(crappy) remove the cat or downstream sensors, and you get a check engine light.
In addition, the OBD-II system has a gas tank evap. system monitor(Ever have a check engine light because the gas cap was left off?) without all that crap, you get another code and a check engine light also.
"Never attribute to malice that which can be adequately explained by stupidity, but don't rule out malice."
My other car is also a Jeep,.. and so is the other one,.. and that one too. How many do you own before it's "a herd" ?