The what logo is a label question has been well defined by @derobert already — brand (and trademark) associated with the marketing of sound recordings, no need to keep saying that we should not include non label logos. ;p
This is quite simple to understand.
No problem for me to include them all (those that are not series logo, artist logo, hairdresser logo, etc.), as it’s the right way to disambiguate this edition from another one.
I’ve came across, several times, editions that would almost only change in one label displayed or not.
Instead suggesting a relationship (they are not easily spotable, they make editions hard to select from release group page as they are invisible), let’s stay on the previous suggestion of having a checkbox primary label (main label) and primary catalogue number (main catalogue number) that would be for label experts (not me) to use without removing our work, us, non experts.
Well, except if label field is completely dropped in favour of relationships and that those are displayed on release group page and all the pages that show list of releases (searches, artist releases, label pages, etc.). 