Whenever you lookup a release it outputs the cover-art-archive and asin tags, so this is basic data no inc paramter is needed

i.e http://musicbrainz.org/ws/2/release/1b41196f-177f-40b7-ab44-c4e1aae7fb9e

Therefore, when you do a releasegroup lookup and include releases it should output the cover-art-archive and asin for each release but it does not

i.e http://musicbrainz.org/ws/2/release-group/cc24d8c1-c004-3aac-8866-2952847992dd?inc=releases

This is not just a consistency issue, there is a real usecase why this info is required. The addition of cover-art-archive tag now allows us to check if a release has cover art before we waste time and resource looking up a possibly non-existent cover art on CAA. Now there are many versions of a release in Musicbrainz and cover art has to be separately associated with each one so if there is no coverart for a particular release a falback option is to use coverart for a release in the same releasegroup. But currently we would have to do a lookup for each release in the releasegroup to find out if it had coverart, which sorts of take us back towards how we were before the cover-art-archive tag was added.

There is certainly nothing in the webservice doc that explains the difference, and I dont think the small overhead of one more database lookup and tiny bit more data is sufficent reason to omit, especially as the issue with the ws is when large amount of data needs to be rendered and returned rather than the db lookup required for that.