It'd be useful for artist credits to have MBIDs (and associated redirect tables). It's difficult to refer to something that has no public identifier. If you want to identify an AC outside of MusicBrainz, you have to either find and use its internal row ID (which is only useful if you have a copy of the database), or copy it in full, as it's otherwise identified only by the sum of its parts (artists, credited names, and join phrases). Robert Kaye has mentioned that this would make his ListenBrainz recommendations work easier, as "tracking arrays of artist ids is not fun."
Having a public ID would open up the possibility of useful features in the web service. We could for example support looking up recordings / releases by "Queen & David Bowie" rather than each of the individual artists only (or performing a search query by text match only). A materialized table + GIN index could allow "fuzzy" matching against ACs that only differ in credited names, join phrases, or artist order.
We've been moving towards ACs becoming more entity like, recently adding dedicated pages for them. Giving them MBIDs means we could allow minor edits – fixing a typo, changing an incorrect artist – and then add a redirect to the new MBID. (The idea being that ACs themselves would remain immutable.) Suggestions like MBS-11339 would become feasible, as "editing" the AC would no longer involve breaking the URL. We could even add redirects in cases where the last instance of an AC is being edited.