Why does update-manager no longer prompt for the user's password?
As of Ubuntu 11.10, update-manager no longer prompts for the user's password to apply updates. This was decided to improve usability and to make it easier for users to apply security updates and therefore increase system security. The rationale is as follows:
Like in previous releases, by default only people in the admin group are allowed access to perform security updates.
Only updates for already installed software can be applied without a password. Installing additional software still requires people to enter their password. The password prompt had become an irritant for some people such that they would just press 'Cancel' instead of installing the updates. The password prompt decreased system security for those users.
People that did dutifully apply updates became conditioned to enter their privileged password perhaps daily. When the user is prompted for the password, it should mean something and the frequency of update-manager updates meant that some people no longer thought about why they were entering their password. For these users, the password prompt had the potential to reduce security.