Going back to the OP's question, while I am not an expert on historical WR services, I think you will find that Slough used to be a more important "Inter-City" station (at least in relative terms, relative to the number of trains that is) back in the 60s and maybe 70s. For example, it was the original station for the connecting bus service to Heathrow, before BR concentrated that traffic on Reading.
And, a bit like, say, St Albans on the Midland, when local services were less frequent, Inter-City stops helped give the place a decent service for the era.
But as local service frequencies improved, and with the growing importance of Reading as the outer-London changeover hub, management felt that time penalty versus the extra traffic generated by having a stop simply wasn't worth it, especially on 'flagship' services to Bristol, the south west and South Wales.
Worcester and Hereford have always been the least important Inter-City services on the Paddington palette, so they've 'held on' to the Slough stops - but presumably GwR management has decided that the penalty involved is more of a minus than previously. I presume removing the stop also means increased utilisation of the potential capacity on the fast lines between Paddington and Reading - so another win there.