Yeah, it's supposed to be temporary. When it was introduced (by Steve?), he was talking about the context of using someone else's computer and not wanting to have to worry about your cookies and history showing them your shopping carts and e-mails and such. Though there's obviously the Porn Mode aspect as well..
At any rate, if they thought people were going to use it as a permanent setting, it'd probably be in the Preferences, not just a menu item.