Ported vacuum for the distributor advance has the added benefit of the ignition timing remaining steady regardless of how the engine is loaded at idle. So long as the throttle is shut, the ignition timing will stay the same.
Not so on manifold vacuum. The engine idling in neutral makes a lot of manifold vacuum and advances the timing, raising the RPM. Put the transmission in gear though and it loads the engine, which reduces the manifold vacuum, retarding the timing, which slows the engine down more, which reduces the vacuum more...
Engines that idle high in neutral but fall on their face in drive are often engines using manifold vacuum for the distributor advance.
'72 Jeep Wagoneer Custom, 360 V8
I love how arguements end as soon as Ristow comments. Ristow is right...again.