I'm thinking enterprise setting/coffee break scenario here. 5-10 minutes is reasonable, 30 minutes is an insanely long amount of time to leave a work computer unattended and wide open to any passerby to fiddle with. The default setting should be closer to a secure setting. Users who don't need security (ie: home users) can disable automatic screen locking or prolong the timeout.
This is similar in spirit to a no open ports/insecure services default policy, or a default policy of asking a password to log-in (ubuntu does offer you the option to autologin though).