One thing to watch out for is that a process running as "nobody" has permission to manipulate (e.g. kill) other processes running as that user. This sometimes includes system processes. Allowing any user to run arbitrary commands as nobody is technically a privilege escalation, and therefore should be avoided.

A single-purpose user with its own group would have this problem only to a lesser degree (you'd be able to mess with other users' "up" invocations, but not any system processes).