I disagree. I can't tell you why my brother said the doors were in the wrong place, but the way I see it nobody should be without a seat on regional and regional express services. Nobody ever, even in the peaks.
That would be ideal, but it is not realistic.
Suburban services yes, a small standing allowance in the peaks maybe (as long as nobody has to stand for more than about 10 minutes) but not on regional services. Given that nobody should have to stand, the use of suburban units (eg. 170s) with wide doors providing room for standees at the expense of a comfortable seating layout on regional-express services (eg. Cardiff-Nottingham) is morally wrong.
The purpose of wide doors at thirds is not to provide for standees, though it does that incidentally (you provide well for standees by providing door standbacks and 3+2 width seating with a 2+2 layout, like the Class 700 does). The purpose of wide doors is to ease access on trains where there will be significant on-and-off passenger changeover at many stations, which is true of most UK regional express services, and certainly true of most ATW services where there is not really one single centre everyone is travelling to.
So no, it is not "morally wrong". It is a very good choice for those services. Class 170s or 172s would be ideal for near enough all ATW services except Gerald.
FWIW I do like classes 156 and 158, but loading and unloading is *very* slow - that's why TPE, which itself is a regional express operation despite having ideas above their, er, station, chose new units with doors at thirds, i.e. Class 185.
Yes, only Gerald is intercity. However, even that also serves local markets (certainly northbound) so really is only intercity because it has intercity facilities (hot meals). If they had used a 175 on the same service from the outset, unless there was a clear demand for longer trains and fewer stops, it would be regional-express and (having not been treated to the intercity-spec train) I would have had no cause for complaint.
Yes, you can debate if it should exist at all - personally I don't think it should, fun though it is, as it provides for a really small market and costs a packet with no significant social benefits for the money unlike a rural branch line. The big rail markets in Wales are North Wales to Chester, Liverpool and Manchester (and a lesser extent London), Cambrian to Shrewsbury and Birmingham (and again to a lesser extent London), local services into Cardiff and GWR from South Wales to London.
I see the key difference between regional-express and intercity services as the level passenger demand; if it is enough to justify hot meals then it is intercity, if not it is regional-express. Also, the proportion of the route doubling as a local service (with calls at all/most stations) may be higher on regional-express, but that doesn't stop the likes of Gerald and the afternoo Pembroke Dock to London on summer Saturdays being intercity.
I'd say that's a bit flawed - a regional service is an all stations stopper (a Class 150 being a perfect example of a unit designed for that usage), a regional express a local service that stops a bit less often but still primarily carries local flows (most ATW services are this), an inter-regional service is maybe what you are talking about but few exist in the UK - XC is probably most of it, then InterCity is about big city to big city with almost no local flows at all (you'll get the odd people doing Cov-Birmingham or Stockport-Manchester Pic but if you banned those flows from VT nobody would be significantly disadvantaged, say).
If the Pembroke Dock service was a DB service, it would be "RegionalBahn to Cardiff Central, then InterCity to London Paddington". There are a few like that in Germany, the once daily Hamburg-Berchtesgaden is a good example. Similarly, when XC used to serve the local stops like Barry Links around Aberdeen that would, by DB parlance, have run as "InterRegio to Edinburgh, then RegionalBahn to Aberdeen".