Stopping Penzance services at the likes of Castle Cary has always seemed wasteful, as those seats are unlikely to be required again for the rest of the journey. There is not a lot of intermediate traffic to be picked up on westbound services - Westbury provides some connecting passengers from Southampton etc, and Exeter from both connections and originating passengers, but otherwise the stops are principally for setting down. Even Taunton has always had little focus towards Devon and beyond - commercially it has always looked to Bristol, despite this being further distance than Exeter. I do suspect the new semi-fasts will be pretty empty by the time they get to the latter.
Tiverton Parkway is a strange omission by the new semi-fasts, maybe it will come with time. Frome is a nuisance being on a slow single track loop well off the main line, and risks conflict with Weymouth services in either direction, although it is a significant town (larger than Westbury). Bruton does not have any tradition of London services. I still hold out for one or both of Somerton or Langport getting a reinstated station, both quite significant local centres where the line passes through the centre of town, more than can be said for either Westbury or especially Castle Cary. And maybe, one day in the far future, Wellington.