ATW is short on regional express units, although perhaps not quite as short as I thought. I'm sure I read that 20 diagrams would be used on Cambrian - Birmingahm - Holyhead from May 14th. Now that you have stated it will only require one more than now (up to 16 rather than 20 from the current 15) it seems there is less of a problem.
I think Chiltern fitting power doors to their mark3s shows that some in the industry think there is still a place for LHCS though.
156s might be an ideal unit for that service, or pairs of 153s, provided they have a comfortable interior, but using 150s would be another case of putting them where they shouldn't be.
Neither, it is from a trying to make rail travel a more attractive alternative to the car perspective. Class 150s shouldn't be used for regional express type services, like the Fishguard flyer they are diagramed for and the Holyhead - Cardiff and Manchester services they turn up on in lieu of 158s and 175s. The objective of bringing the Swansea District Line into regular use is also to try and make rail more attractive, although it isn't quite enough to beat the car as it is, but gaining a few extra trains over it might preserve the possibility that one day trains could beat the car from Carmarthen to Cardiff. Reducing the levels of overcrowding on a few ATW services, and providing extra services using long trains during events that generate large passenger flows, could also help attract modal shift, or at least help avoid putting passengers off rail travel.
In this regard, it does not matter whether the extra trains are LHCS or class 158s. In fact, additional class 158s makes more sense for Wales than LHCS except for the fact we can't get any more 158s, but mark2s (that ATW don't even need to lease, except for the DBSOs if they are needed) are available.