It's not just rural Pembrokeshire. I did some looking at timetables a while back and, outside the ValleyLines network, I didn't find many diagrams on the ATW franchise for which a 150 would be suitable, If I remember correctly, I found a grand total of three diagrams where a 150 would be sutiable. The other places where 150s shouldn't be used, such as the HOWL, don't have much 'payback' either, but that's no reason not to try and specify a franchise that, at the very least, requires replacment of 150s with something more appropriate if/when that something becomes available.
150's in fleet numbers are not going to be free to work West Wales or anywhere else hypothetically until the Valleys are wired and a suitable electric fleet brought in this could be 2020 or beyond and the humble 150 will well and truly be close to life expired by then. The UK electrification program will not provide a magic answer to rolling stok on non wired lines as the scale of the program is not large enough and the timeframe to long to avoid life expiration and growth issues elsewhere.
Stadler GTW would be ideal for West Wales locals, Heart of Wales, Cambrian Coast, locals on Cambrian mainline and Conwy valley. A 25 strong fleet would be sufficient, the Conwy Valley provides the logistic headache.