It can be cheaper to travel via the HoW, and from Llanelli there's actually very little difference in journey time between the two routes. I personally find the How more interesting, so while I;d agree that the through market between Swansea and Shrewsbury is pretty small, there will be those that just want to save a bit of money or just have a pleasant train ride whether they count themselves as enthusiasts or not.
I'm stating the obvious, but the higher frequency of services along the Marches and the higher populations served means that that route is always going to busier than the HoW. On the latter line the majority of users when I've travelled have been between intermediate stations. Much fewer have done a full trip, but there have been some, at least to and from Llanelli, where i usually board or alight.