Well, this is a kind of truism - but yes - if they ran, and ran roughly on time 95% of the time (which, sadly, the HoW all too regularly fais to do).
I might agree with you, but why?
Er, this is confusing.
a) Are you are advocating some sort of regular Cardiff - Llandeilo - (or even) Llandovery or Llandrindod service?
Or reopening* the Swansea District line for some sort of Carmarthen - Cardiff service?
Or both? 'Cos you can't serve Carmarthanshire from Cardiff by going oop north at Hendy.
* Yes, I know the odd service does run that way each day, but it is effectively not open for passenger services. Fut for the record, I think it is a sadly under-utilised piece of infrastructure.
However, this is all straying into the sort of space that I fear will result in the red text of wrath from a mod soon appearing.