I wonder if request stops would mess up timings - what if a train arrives at a request stop early and there is nobody around - are the crew meant to wait to the advertised/WTT time or do they just carry on through? What if there are several request stops in a row, such as between Bangor and Holyhead or on the Heart of Wales line?
There was an article about this is in an issue of RAIL. ATW confirmed that request stops can cause problems and cited the Heart of Wales and Cambrian Coast lines as an example. Where there are several requst stops in a row trains can arrive early at the nxt compulsory stop, and they are then required to wait until the booked departure time.
I can't remember whether anything was said about trains passing request stops early, or whether anyone had claimed to have missed a train at oen of them because the train had arrived early.