The real world is that they keep the brakes on longer when you don't require this and don't work well enough when you do need some kind of "retainer" on steep grades . Grade control valves were only EVER a PITA when they were not maintained properly , when they were they gave you choices fixed chokes don't . We KNOW for a fact that in other States examiners were NOT carrying out PROPER grade retention tests because even 115 km/h rated wasps can't build nests in them on the fly . Too many wagons brakes would not release with the grades in IP . We used to have a few five packs and or high speed flats on the front of the Road Railer trains and put the grades in at Newbridge heading towards Sydney . All too often we'd get dragging brakes but only after the first application . Why ? Because the vent port was blocked and it happened too often in the run from Perth/Adelaide to Sydney . The fix ? Shove an oxy tip cleaner through the hole .
Fixed chokes are dumb as in a simple device with compromises . Another dumb compromise might be fitting a locomotive with a smaller engine to stop it slipping rather than applying an "intelligent" traction control system . Had grade control valves been designed to vent some air through the EX and IP ports they would have remained clean and servicable . These days the cheap dumb fixed choke means you stand around losing so much time when shunting wagons it isn't funny . You didn't see fixed chokes fitted to DMUs/EMUs because they occassionaly ran down steep grades did you ? And before you throw in the EP spanner what do they use when the EP fails ? Yep a 26L emulation and they run a lot slower when they do .