As per the comments on revision 13218 of musicbrainz_mmd-2.0.rng http://bugs.musicbrainz.org/browser/mmd-schema/trunk/schema/musicbrainz_mmd-2.0.rng?rev=13218 "Added the disambiguation element for release group, this is due to replace the badly name comment element but the comment element will not be removed for six months in order to preserve backwards compatability" This was written on 2011-07-26, some 9 months ago at time of writing.