I believe the pumps were added more for practical reasons.
Edaville had stripped most of the brakes off of the cars as the freight work gave way to passenger operations. The passenger cars were a mishmash of straight vacuum brakes (B&H cars) and automatic air brakes (the SRRL passenger cars) or built inhouse without brakes, so standardizing the brakes was probably a low priority since they operated in a very insular manner (no "public" road crossings, limited down grades, etc). You also had the issues of kids closing brake valves, pulling emergency valves, turning brake wheels, and so on.
The other thing is that by the time that the air compressors went on, parts for the vacuum and steam cylinder brakes would have been a lot harder to come by. 9" compressors were very available in the 50s as locomotives were being scrapped so the conversions could be done fairly cheap from the standpoint of the physical parts. Plus if you strip the steam brakes out of the Baldwins you have double the parts to fix the brakes on the Monson engines if need be.
I have a feeling that if Blount had lived automatic air brakes would have been installed on all of the cars at some point. There's actually a 6" compressor for a Monson engine stored somewhere in the MNG boxcars. It was it was likely seen as an expense that they could just forego and operate around.