To give TurdFerguson the benefit of the doubt, when I saw his statement Our posts cover everything that matters on this subject it read to me as if he was just saying that pretty much the whole range of opinion has been covered by the various posts in this thread. That might or might not be what he actually meant, but it's how it hit me.
One point that hasn't yet been covered is that all ported spark advance is not the same. The distributor vacuum advance hose always left the factory connected to the spark advance port on the carburetor, but where the port went from there on in varied according to the vehicle configuration. Have you ever found yourself looking in the FSM or FPC at the two carbs for manual vs. automatic transmission for any given year/model/engine, and found yourself unable to find any difference in specs? Same jets, same step-up rods, same choke kick adjustment, same float setting, same venturi size, etc., right on down the line, but different manufacturer numbers and Chrysler part numbers for the whole carburetor? In cases like that, the difference is usually the location of the inner end of the spark port in the throttle body with respect to the throttle plate as adjusted for the factory-specified engine idle speed at the factory-specified base ignition timing. If the port is above the closed throttle, there's no spark advance until the driver steps on the gas, at which point the advance jumps up and then tapers down as the driver presses the accelerator further down. If the port is below the closed throttle, there's spark advance at idle which begins to taper down as the driver opens the throttle. In a 1978 emission control and driveability manual published by Chrysler, they go into some detail on this, complete with cutaway images of two throttle bodies, one with each type of spark port routing. There's specific, documented evidence of the two types of porting as early as 1975. I've seen differing spark port locations in the throttle bore as early as 1960, though I don't have specific info from Chrysler saying so.
So, some carburetors are set up such that the distributor vacuum advance sees full manifold vacuum even at idle. This, according to factory service literature, was done to give better driveability in certain vehicle configurations with automatic transmission. And there's at least one vendor of performance ignition systems for Mopars who advocates running unported manifold vacuum to the distributor. So I decided to try an experiment on my '73. This car has a Carter BBS carburetor, a 1976 distributor (the one for the 2bbl engine, with the good advance curves), and no EGR. The carburetor's vacuum advance port does not have vacuum at idle. So I capped that port and connected the vacuum advance to manifold vacuum. Left it there for a couple of weeks to get a good, solid idea of the changes. One immediately obvious change was an increase in idle speed; I had to back off the idle speed screw. No surprise there.
Driveability did not improve, it got worse. Specifically, the car's pickup sagged slightly but detectably immediately after the 1-2 and 2-3 shift, and light-throttle acceleration in 2nd and 3rd gear (well short of causing the trans to kick down) was noticeably mushier. Once I put the advance hose back on the carburetor and readjusted the idle speed, the sag and mush went away.
My guess is that the driveability improvement with manifold vacuum spark advance is specific to the underhood configuration. Mine differs substantially from the setups on which manifold spark advance was used; I have no EGR, my base timing is about 8°, etc. Still, I can't help wondering why I remember seeing manifold vacuum at the spark port of various more-or-less stock carburetors on my '62 Lancer back when it was my dad's car. I'm pretty sure the vacuum porting particulars depended in the pre-emissions era on manual/automatic transmission, but back then I think it was manual-trans cars that got porting closer to manifold-vacuum-at-idle.
FWIW, YMMV, BYOB.
