On releases we know if it's official release or not.
This information would be very helpful if it was available on release-groups too.

I think it's easily doable because when you go on an artist page (on MB website), you can see all the official release-groups only, and there is a "Show all release groups instead" link to show unofficial release-groups.

Providing an attribute status ("official" or not) on release-group would be very useful.

That way we could avoid getting all unofficial release-groups when browsing an artist discography...

You can see topics about it here:
http://forums.musicbrainz.org/viewtopic.php?id=5618
and here:
http://forums.musicbrainz.org/viewtopic.php?pid=31533