There are at least two third-party implementations of constraint checks – HarvestTemplates (code) and OpenRefine (code) – which check constraints on some data before adding it to Wikidata. It would be nice if we supported this in WikibaseQualityConstraints, so that third parties don’t have to re-implement everything.
Technically, this means adding support for passing JSON snippets (snaks, full statements, or full entities?) instead of entity or statement IDs into wbcheckentities. (I think that makes more sense than a separate API module.)