If you have a system where you have a bunch of users executing their own CGI scripts, you probably want to do this, to limit the amount of damage a user can do. Take it a step further and put resource/file system limits on users as well.

Just be advised that if you're allowing users to run their own CGI scripts, you can forget about security. Odds are, they've got some insecure stuff up there, and if your system is going to be compromised, this is definitely the way I'd try and do it. Except now instead of running as a 'nobody' user or a restricted web server user, they're running as a legitimate, real user on your system, which could make it a lot easier for them to get further.