You shouldn't do that. There some reasons for such behavior: 1. If you do "emerge jabber-server" it installs new jabber-server, but then removes old, and removes jabber group and user. 2. If you do "emerge unmerge jabber-server" there could be left some files and directories owned by jabber group and user. And when you then add new user with adduser the uid of new user can be equal to uid of old jabber user. Sometimes it can be not very secure.
So i should just leave the user and group there after it removes it, and just show an einfo to remove them manually?
Yes. I thinks it will be better. Many othe ebuilds don't remove users and groups after umerge. Look at apache.ebuild for example.