Firefox does it better, allowing you to set a 'master password'. A fresh launch of the browser can't autofill fields until the master password is provided. The settings page for saved passwords won't show them in plaintext unless the master password is provided again.
It's not very onerous and it prevents quick, casual swiping of passwords. (Sophisticated attackers could navigate to a fresh login prompt and use DOM-manipulation... but that's also more time-consuming and doable by far fewer people.)
1. Safari stores its passwords in Keychain (OSX's built-in password manager), in which a master password (the account's password) is required to get cleartexts. Safari itself does not give direct access to the passwords list.
