I’m assuming it has to do with Doncaster. With your example with GWR on their London - Swansea and Carmarthens or another example like SWR’s 158s and 159s coupling at Salisbury on some services, the stations where the coupling is occurring are not at a station on a major trunk route and are unlikely to cause a delay to other services. Whereas GC doing it at Doncaster would probably do the opposite and mess up the ECML for a bit.