Great idea. Just what I would have liked to use. But because I was specifically looking for Rights on Category level it did not answer to my needs. Users can choose any category to submit an article regardless of the specification of only one single category. Realy too bad and I hope you can find the time to modify the code so the selective category setting works. When that works I will grant you an Excellent!!!
UAM v0.17 and later should address this issue. Users with 'Author' or 'Editor' rights (as defined in a default Joomla 2.5 installation will be restricted to the given category when creating articles. 'Publisher' and superusers can still change category if desired