well here is the point. When you upgrade, any packages that are not on the disc are unemerged. If I just emerged the new sabayon I could keep what I have already emerged instead of starting all over.
2. change flags & re-emerge it ALL with new flags
3. re-emerge all stuff I had before the fresh install
4. put back all the files/music/docs bla bla bla where I had it before fresh install (usually about 2 DVDs worth for the basic stuff)
5. re-install video games (not in portage - EX: NeverWinter Nights)
As you can imagine, this takes a long time alone to install it and re-emerge it all. (it can take over 2 days) So why not just emerge it all from the beginning with the flags I already have set keep my files without having to back them all up and put them back on the HD later and NOT have to re-install my games and set permissions etc. etc.
maybe we could add some use flags like "nognome or nokde" which would leave out those packages that are related to those environments.