the intake vacuum might rise up enough near maximum RPM's to start the vacuum advance
True - this is potentially dangerous.
A high vacuum level under power will also shut down your power system (pull the metering rod down, or close the power valve) and lean it out.
The choice of ported or manifold vacuum is also a way of "cheating" the throttle blade angle position at idle. Depending on the cam, PCV-based carb or not, etc. your idle setting may place the disc edge too far into the transition slot for best response, giving a flat spot off-idle.
Increasing the idle spark sometimes allows the disc to be re-positioned where it should be - about .020" to .040" into the transition.
Pre-PCV carbs frequently don't tolerate the extra air added by a PCV very well, and lowering the idle speed manually causes the disc to retreat too far away from the transition slot. In this case, more spark may make the problem worse.
Horses for courses.
