You still have to include time for possible stops at request halts although it is unlikely you'd ever have to stop at every one. That makes planning regular interval services with precisely timed passes difficult, and in practice means trains will hang about at passing loops longer than strictly necessary. That in turn extends journey time reduces stock utilisation etc etc.
Fair comments - I think the main issue would be between Eggesford and Barnstaple as Umberleigh is the only booked call (by request) on most trains. Kings Nympton gets some random calls thoughout the day, so a train booked to call at every station from Eggesford to Barnstaple on its return would probably only be able to call at Umberleigh. The next train would do the opposite and would call only at Umberleigh, and then every station on the return back to Eggesford. You would need a time for every booked station call, but other lines (HoWL, Cambrian Coast) assume that the train won't stop at every station. Although some trains do have dwell time at Crediton and/or Eggesford, it doesn't help that turn-around time at Barnstaple is fairly tight.
All stations (bar Chapelton) are at least at 1,500 per annum for passenger numbers now - There are stations elsewhere in the UK that get a similar usage with a more frequent service, and with passenger numbers currently at their highest in 10 years at Crediton, Yeoford, Morchard Road, Lapford, Portsmouth Arms, Umberleigh and Barnstaple, this shows now could be a good time to capitalise on it.
(Obviously other projects in Devon would come first, like making the Paignton locals half-hourly, possible all year round Okehampton service but with the Tarka line, the stations are there and maintained, the trains are already running, Tarka line advertising is in place all over the GWR network - It seems like a relatively low cost way of encouraging more people to travel by train.)