I can't think of a good reason to use NetBSD [1] over Linux [2] for any architectures that Linux supports. I guess licensing is an issue for some companies. If there's another reason, the NetBSD project needs to communicate it better to attract new users.
You can use both - you can run NetBSD in userspace under Linux http://www.netbsd.org/docs/rump/ - eg to run userspace filesystem drivers or a userspace IP stack... you can't do that with Linux.