I believe it is occasionally asked for for flights originating in some slightly dodgier countries (or vice versa).

I have been asked for it twice, once by an OLCI machine somewhere, and another time at the gate. Both were locations known to be slightly dodgier in terms of credit card fraud, but they were also both before I started crediting my flights to BAEC. It wasn't an issue since I had the card with me anyway.

I cannot recall being asked for it since I started crediting my flights to BA but it coincided with the time when I started consistently using the same card for BA bookings too, so the BAEC thing may well be a coincidence.