In this post on an Ubuntu forum thread someone poked around the package contents to see what the addin's source actually does when enabled. It lists the following functions:
- enables usage of Ubuntu plugin finder service
- allows localization of extension descriptions
- sets several browser defaults, like printing, look-and-feel, disables checking for default browser, locale matching with OS, etc.
- adds Ask.com search plugin
One undesirable side effect is that it seems to interact oddly with a number of other extensions1 such that you can not add their toolbar buttons to any toolbars, even on the default theme2. Disabling the Ubuntu extension re-enables adding these buttons to toolbars, hence I currently have it disabled3.
1. most notably FlashBlock and TabMixPlus in my case
2. some people suggest the problem is related to theme choice, I have found this not to be the case
3. though I have not completely uninstalled it yet, in case it does turn out to be needed for something else and I want to reenable it with least hassle