To answer some of the points raised by moonarrow458:
Potters Bar Garage dead runs: It has to be understood that what TfL wants is what TfL gets!! Metroline have (in the past) offered to schedule dead runs in service on Route 384 234 (when the route ran from PB Garage and breaks were taken at the Garage), especially after Route 383 was cut back to Barnet. Because we couldn't offer a regular service (every 20 minutes in the daytime, but no buses northbound before 09:00; no buses southbound between 07:30 and 10:30; no buses in the peak hours) . . . all because all buses were in use on the basic route (obviously). Any extra trips would cost an extra bus and two extra drivers, and TfL didn't want to pay as the benefit would be to Hertfordshire residents.
If Herts CC were prepared to contribute financially, maybe the answer would have been different, but at the time Route 84 was still running . . . it's one of those cross-boundary routes that needs co-operation to succeed. I will just point out that the current Route 84B (funded from Hertsmere DC monies) runs hourly, and after an initial flurry of passengers, now runs reasonably well, but also rather empty apart from a few well-used journeys . . . I'd suggest that what demand was there has evaporated now.
Route 43: Guilty as charged, M'Lud!! When I worked at Metroline as the Schedules Manager, I had a dislike of buses running dead on Central London routes, especially if they could add to the overall service. At the time (early 2000s?), TfL didn't have a lot of expertise in planning timetables, and if we could add such trips in for no extra cost, we would be allowed to do so. If you look at the schedules from these times, there were NO dead trips to/from London Bridge . . . the basic schedule required them, but I ran them in service and hid the 10 minutes extra driver cost per trip in the basic tender bid . . . so TfL did pay a little something, but in the scheme of things, barely 5/8ths of nothing!!
The Friern Barnet trips are something different . . . TfL require a bus past Highgate Station northbound every (say) 6 minutes between 07:30 and 08:30, but because HT Garage is at Archway, the extra trips required can start there . . . the alternative would be buses leaving London Bridge every 2-3 minutes at 07:00, with extra inbound trips, and 3 extra LB trips costs one extra driver.
Route 237: Also Guilty!! Back in Armchair days, when Metroline bought the company, buses ran dead to/from Shepherds Bush and Hounslow Heath, 'cos that was what TfL specified. When I got hold of the schedule, I ran as many as possible in service . . . again, the extra cost was miniscule, and I suspect that no-one at TfL noticed!!
Route 189 and Route 4: These simply avoid starting the bus at the northern terminal . . . the saving on the schedule is tiny, but once they're there, they can stay!!
I'll just make the point that TfL usually don't like these short journeys, as it makes the tendering of the route difficult, and (for example) if Route 237 was won by London United from Hounslow Garage, then some of the short journeys would need to be withdrawn, which would inconvenience passengers. Indeed, when the route is tendered, the specification will withdraw the shorts, to make the base specification to all operators . . . the successful operator will have the choice of re-instating them. For that reason, most operators don't bother . . . it means recompiling the schedule to add them back in, which all takes time.
Sorry, mods, for the rambling, but honestly . . . the questions asked demanded a detailed reply.