From http://bugs.musicbrainz.org/ticket/4061

There is currently no way for editors to tell if a release has been added to someone's collection. I believe we need this information to ensure good decisions are made about removing and merging.

If editors aren't aware that a collection has been added to someone's collection, they may be more inclined to remove that release than they would otherwise; potentially due to very bad PUIDs, historical track order changes or a release being very new.

I suggest this go on the release details page.

I'm not sure which license this collections data comes under (are people's collections private?) but I guess it probably doesn't need to be available from the web service.