Adding, let alone tuning vacuum advance is just about the same as choosing and tuning a carb for most guys. It is just over their head.

For a car that cruises at light load, steady throttle why wouldn't you want vacuum advance? What could possibly be the down side? I agree that on some combos the advantages could be modest but there would still be advantages. It would seem that the biggest hurdle to widespread use of vacuum advance would be the misunderstanding of it's benefits and the lack of ability to tune it.

I use a hand held vacuum pump to test and tune my vacuum advance. You can hear the difference in the exhaust note as you add advance at cruise. I find that finding a good amount of advance is not difficult at all. The hardest part is making the necessary changes to the advance stop.

It would be nice if there were some more definitive data on this. Maybe an episode of Engine Masters?