If the slot is unused, it'll allow the WAL that's being held by the slot to be removed. It'll also unpin the catalog xmin to allow autovacuum to clean up dead tuples in the catalogs.
This doesn't explain how the system got into this state. For that it'd really be necessary to see the steps taken during setup. BDR tries to protect against attempts to replicate-from-self. Presumably there's an oversight in those checks. If you're able to reproduce this state I'd like to hear details on how.
