I do wish to comment AK on what was a bit of a bugbear and thank you for the opportunity!
Llanidloes is 8 times bigger than Llangurig and is a proper town rather than a small village but does not find itself on major road signs whilst Llangurig which is 6 miles away , as you have pointed out, is advertised from many miles away.
The reason for this is that Llangurig is at the junction of two trunk roads, the A470 and the A44, whilst Llanidloes is not. Llanidloes no doubt loses out on passing trade, especially since the bypass was built but probably is better off overall as many ne'r do wells will just go flying past!