Sometimes I think having more options helps end users but at times hinder Musicbrainz.
If the release has art for any other source other than CAA for the release the user is discouraged from contributing.
It all depends on the goals of Picard and its overall relationship with MB.
Likely ease of tagging outweighs encouraging Musicbrainz data additions but with the new releases being added to Musicbrainz outpacing the cover art added to any release, I do worry about the long term future of CAA at times.