Having the doors at the ends of the coaches reduces the gap over which people have to step to the platform at curved stations and therefore increases safety.
So, someone travelling say from Manchester to Bristol gets a proper Inter City train (if you can call a Voyager that), running at high speeds and only stopping at major centres such as Birmingham and Cheltenham. Someone travelling a similar distance from Manchester to Cardiff will get a train with doors at thirds letting in the cold air and making numerous stops at rural stations such as Church Stretton and Craven Arms. (It is the same on the line from Birmingham to Cardiff where often overcrowded 2 and 3 coach 170’s with doors at thirds make numerous stops. This will even get worse when Worcester Parkway opens = so slower service and more over crowding). So, obvioulsy Cardiff is a less important place than Bristol and can be linked to major cities in England with trains that double up as rural stoppers rather than proper expresses.
I think that The Marches line could do with expresses as well as stoppers - especially if a new station were built at Caerleon. (It is desperately needed at Caerleon due to the poor road links with the M4). Perhaps the disused platform on the east side of Abergavenny station could be brought back in to use for stoppers from here to Cardiff? Abergavenny could also do with a car park serving the A465. As things stand, people are using the lay-byes and going down a bank to access the station from the east.
I note that Transpennine Express are getting rid of their 185’s (with doors at thirds) and switching to proper Inter City Trains such as the Hitachi 800’s for the longer distance runs to Scotland.
Mention has been made of the time it takes to off load/load passengers with end doors. Surely, this problem mainly arises due to the overcrowding? Hopefully, this will not be the case once sufficient capacity is provided? I can understand having doors at thirds on the frequent stopping services in the south Wales valleys.