Comment 21 for bug 252140

Since it was dismissed from Maverick one can take the time to analyse the unofficial repos problem.

It is possible for any user to add unofficial repos to the system and though everyone knows the risks of using them, after they are added to sources.list there is no clear distinction between them and the official. So the problem here would be an OFFICIAL Ubuntu package adding UNOFFICIAL repos and not the repos themselves.
One easy way to solve this while, improving Ubuntu tweak, is to merge the software centre into Ubuntu tweak and adding an unofficial repos section that is clearly marked and that pops up a message like "The repositories in the unofficial repos list were not in any way revised by Canonical or the Ubuntu community and contain pre-release software. Be aware that using packages from these respos you might lead to decrease system performance or other problems."
To ensure some degree of security, duplicated applications would not be allowed, i.e. since Firefox is already on the official repos only non stable versions of Firefox would be included on the unofficial repos. Also only the official applications repos would be included, i.e. Firefox would never come from a repos that is not maintained by Mozilla. In case this is no possible, another warning would pop up to tell the user that neither Canonical nor (in this example) Mozilla are responsible by that repo.