Surely the point is that if you try to stop or slow a train using just a locomotive air brake you're going to cook the brakes.
If you tried the same with just a rheostatic locomotive brake all that heat will (or at least should) get safely dumped to atmosphere by the resistances with no risk of overheating.
Obviously train brakes have evolved over the years and we no longer have unbraked freight trains on the railway. It wasn't like it when I joined BR in 1978 and in the 1980s when things were the most interesting for me. I became a TOPS clerk in 1980 in the West Mids and there were still "loose coupled trains" into the mid 1980s. At that time freights were classified 4,6,7,8 and 9. Only class 4 and class 6 were fully braked with 7 8 and 9 requiring a brakevan. This changed at some point in the early 1980s but don't know exactly when. From this point classes 7 and 8 became fully fitted too with just class 9 relegated to unfitted status. Class 6 became 60mph, class 7 - 45mph and class 8 - 35mph.
With loose couple trains there was minimal braking and I recall a ride on Saltley target 76 where i had officially arranged a brakevan pass in order to "do" the Kingsbury branch. Target 76 was a pair of 20s and we went off the shed over to Bromford yard then propelled a brakevan to Hams Hall power station onto the rear of 32 HUOs, then ran round onto the front. The train would officially have run loose coupled as the only brakes were on the two class 20s, and the guards brake in the van. On locos in multi the straight air brake only applies to the loco you are driving from so it was best practice to use the auto brake to apply the brake on your "slave" multi d loco but this wasn't always applied depending on the individual driver.
We took the hoppers up to Daw Mill colliery over the weighbridge, then loaded them under the rapid loader, weighed them again then set off down the hill to Whitace Jn then into Hams Hall. There was a very slight drizzle breakfast time on a warm morning so it was a good rail for loose couple trains and sure enough on the decent through Shustoke they "picked their wheels up". The experience of the driver was enough to juggle the brake valve and get control of the loco brakes again, and with the brake hard on in the van at the rear to keep the heavy and unbraked hoppers tight on the rear we didn't come to any bother.
At Hams Hall we ran round and took our brakevan to Kingsbury then u the branch to collect traffic from Baddesley then making up at Birch Coppice before running the "full digger" back to Lawley St "A" box to stable.
Regarding the straight air brake - yes, used a lot on unfitted trains to gather the wagons up, but also remember that totally unfitted trains like T76 that day rely on only the loco wheels for power brakes. A job from Bescot used to be Target 18 known as the Lea Hall tripper. This engine went light or engine and brakevan from Bescot to Mid Cannock, then shunted together up to 32 unfitted coal wagons. These would be loaded by gravity under the rapid loaded ad then marshalled with a brakevan on the rear. This was one of the very few coal jobs that didn't need a slow speed fitted loco and I believe was diagrammed a class 45 at one stage. Wagons would be anything that carried coal, mostly 21T hoppers or minerals but a smattering of 16 tonners too. There was a minimum brake force requirement but the crew very rarely tried to shunt a couple of vac fitted wagons next to the loco and the train ran across the Cannock Chase and into Lea Hall colliery yard adjacent to Rugeley power station. Here the wagon contents from Mid Cannock open cast were tipped onto a conveyer belt and delivered to Rugeley A power station.
As a single type 4 diesel on T18 the driver often used the straight air brake as the loco brakes were more responsive than applying and releasing the auto brake to do the same job.
HTH
