I believe the serial number requirement is new in 2017. Agent told me specifically that it was to prevent re-use when not collected since many agents were not doing so at the airport.
That makes the most sense. I had it with me but nobody asked for it. If the serial number requirement means that I don't have to stop by the ticket counter for a paper ticket, then I'm all for the change. Yeah it stinks losing out on the occasional double dip BXP1...but it's about time they caught up with this loophole.