It also forces >24 hour connections on international routes. If you try some like DTW-ATL + ATL-MBJ with both flights on same day, it will just bomb out. Most likely a work-around related to multi-city searches not checking married segment inventory (and only checking individual flight inventory) and mistakenly offering fares for which there is no married segment availability. If you prevent the multi-city search from offering any connections where married segment inventory would apply, it can't flub it up.