Collections are nice for keeping sets of tracks, which could be used for all kinds of things (like the basis for a similarity-based recommender engine), but they can't store playlists.
It would be great if it was possible to store playlists in musicbrainz. This would basically be an orderable list of recordings.
It would be particularly useful e.g. for syncing playlists from other software (e.g. Mixxx: https://bugs.launchpad.net/mixxx/+bug/1741147).