1971 360 question

Guessing a little bit on the history but that's about right.
It's certainly possible that the 1/2 idle was something they could tell the average mechanic to do when 'tuning' the aftermarket holley 2 bbl or 4bbl. With a basically stock engine could get away with it, or at least be close.

Tuner has written about the low opening power valve myth and how that came about when fuel changes were eating rubber parts. So that ties in there. Drag racers for the most part don't deal with part throttle as long as they can get the engine to go to full throttle off the line, getting the part throttle acceleration fine tuned wasn't considered important.

IMO, what anyone interested in the subject needs to know is that for driving; idle should be moderately rich, off idle should similar or leaner, getting leaner with more throttle. Increasing throttle for acceleration or up hill, or higher speed cruise (interstate) continues to get leaner.

As the engine load approaches the maximim it can produce at a given rpm, that's when enrichment is needed. The better the fuel distribution, the later this enrichment is needed. A very efficient truck engine may only need enrichment when the vacuum drops to 5 or 6 inchs of mercury. A hot rodded engine with intake and carb and exhaust that doesn't have great part throttle distribution maybe it really does need enrichment at 12"Hg. Chevy's 396 with the 3310 came with a 10.5 (p) and 8.5 (s) PV. All of the Chrysler's performance engines that came with Holleys in '67 and '68 list a 8.5" PV. Relatively speaking the Chrysler's were a bit better (and arguable a little less radical).

Vacuum advance goes completely away about the same time enrichment is needed. So that's another place hot rodders can get the tune messed up. It's best to test for a part throttle dead spot with the vac advance disconnected and a mechanical advance that looks something like the factory's. Ideally a nice open road when one can squeeze on throttle from 50 mph or higher, then do it again but squeeze deeper.
If crusing with 16", should get more acceleration as throttle increases and vacuum drops. It's a little tedious. it's why we say tune to the point that you are satisfied. Some people want to squeeze all they can out of a setup, and others are happy of the car waddles down to the Sunday coffee and donuts.