If you go to install something via the software center, it installs it system-wide, which of course requires a password.

Now this is obviously annoying, but would also seem to also cause a security risk; if the user gets used to entering his password at any point, he may enter it when he has no idea why the dialog is there, allowing malware to get root access.

Of course, the password dialogs are a necessary part of security. But everything in the default repositories is known to be safe, and a password is required to add non-standard repos.