The Birmingham thing probably explains why there might be a magnetic OSI between the two terminals, but in terms of Oyster it doesn’t make a difference. The OSI is only between the two NR stations, so no benefit for other tube options. The Elizabeth line might depend on how that is classified at Paddington. The reason I don’t think it should be there is that both lines head out West with their first stop some way out from London (Wembley Stadium and Acton Main Line) so I just don’t think people use it. This is backed up by it being the least used OSI in the whole system, apart from the redundant one at Hackney Central/Downs.
At risk of going massively off-topic, I still don’t really understand your fascination with the short-hop Underground fares across the zone 1-2 boundary. As far as I know their existance pre-dates the Oyster system by a long time. They’re not the only special case in zones 1-6 either. There are a huge swathe of journeys using NR from south and south-west London to Vauxhall or Elephant & Castle, then changing to the tube for a zone 1 only trip, where the fare charged is the sum of the two parts rather than the normal NR1-T scale fare. There also used to be some special cases on the East London line where an NR1 fare was charged rather than TfL-LU, but I think the mayor’s fare freeze put an end to those.