Scully, I guess in an environment where people were encouraged to take lesser jobs for the greater good, i.e. to allow more time in field service, there would have been a lot of frugal pragmatism. Back then, conventions and assemblies were a much bigger deal and better frequented than what is being reported in recent times. So the WT would have still got decent kickbacks from the obedient "ones" booking as they were told.
I doubt that that dynamic exists anymore. The big out of town meetings aren't attended like they were, there're plenty of PIMOs who will do the bare minimum, JWs in general are just tired and disillusioned. If ever there was a graph over time showing JWs who book through approved channels versus those who make their own arrangements, it would be most interesting to see how much things have changed.