On the CM5, corefiles tend to be large, and so they tend to consume a lot of disk space, and take a lot of CPU time and network bandwidth to write out.
Most users have no use for corefiles.
If you are using another shell, you can use the command setnocore. Of course the argument to setnocore can be any program, it doesn't have to be a shell. Setnocore is a small program I wrote which adjusts the coredumpsize parameter to 0.