There are plenty of orders for trains that are only used at their initial TOC for a short while: the 350/4s have only been at TPE since 2014. I'm not expecting the scrap at 2020! Having a set of 65 new-build carriages would be useful for numerous reasons.
110mph commuter EMUs will always have a home somewhere on the network, unlike new coaching stock. Even a microfleet could probably find somewhere eventually, e.g. running electrified branch line services in Devon and Cornwall. The 350/4s in particular could be used almost anywhere on the network, regeared down to 75mph for use on inner-suburban services or kept at 110mph and used for electrified regional services on 125mph mainlines.
New unpowered coaches wouldn't have anywhere near the same number of possible use cases. While they may be useful while coupled to a diesel locomotive, any service they do coupled to an electric one would be done more efficiently by an EMU. Even rural routes wouldn't be suitable since the key to rural routes is to increase frequencies and journey times, which will always favour lightweight and short DMUs rather than locomotives carrying any number of carriages.
The only time that carriages are efficient is when there are enough of them to justify a heavy locomotive - since there are no light traffic locos any more. The only such services are those which run at 125mph on congested electrified main lines, and so would be best off with fast-accelerating EMUs which can use every metre of platform space for passengers, or the sleeper services which are already getting new rolling stock. The sleeper vehicles have a different window arrangement to the passenger vehicles, so I don't even see it being possible to convert any new day coaches to become the new sleeper vehicles for the Night Riviera service.
