[davepl]

Here's my theory, take it for what it's worth.

1) Vacuum advance should be ported
2) Lots of people have poorly tuned motors with vacuum leaks, etc
3) More timing crutches those symptoms
4) Running vacuum advance to manifold gives you a bunch of initial timing

So yeah, plugging the vacuum advance into manifold vacuum will give you a bunch of initial timing that will make it idle better, but it's not the right way to do it.

There are SOME things (like TCS - Transmission Controlled Spark) that are just nonsense emissions crap that was a bad idea even then. But vacuum advance is a big deal for driveability (and mileage if you care).