It has happened, on occasion. The X90 coach used to accept regular Oxford-London return train tickets, so if you missed the last train, you could catch the bus. But that was when Go-Ahead, who own the Oxford Bus Company, had the Thames Trains franchise.
We had exactly the same arrangement in place for around six months in 2014 ... with the last bus from Chippenham (22:16) to Melksham and Trowbridge accepting rail returns long after the final train of the limited trial service (20:23) had left. It made sense, and the bus left Chippenham Station with a goodly number of passengers - a mix of the local traffic and rail transfers.
Sadly, the commercial operation of the daytime bus came to an end, and with that the council supported evening service was also withdrawn, saving the council around £20 on each supported evening journey.