If an ignition system is designed to operate with ported vacuum, you can get some bad results using straight manifold vacuum.
Like 4onthefloor said, ported vacuum becomes manifold vacuum after the throttle is opened slightly. When the throttle is closed, there is, essentially, no vacuum.
At idle, there is no need for additional advance. Piston speed is slow, but the fuel mixture still burns at nearly the same rate. Using additional advance may make the egine idle somewhat rough and the typical response is to adjust idle mixture rich to smooth it out. This shortens the life of spark plugs, carbons up the combustion chamber, and can contribute to poor off-idle response.
Upon opening the throttle, manifold vacuum is allowed in and vacuum advance begins to be phased in. If the throttle is opened a lot, manifold vacuum will remain low and, consequently, vacuum advance will remain low (or none). As RPM rises, mechanical advance will phase in, regardless of vacuum. As RPM comes up and the throttle is reduced, vacuum and vacuum advance will increase.
At cruise condition, you will have maximum manifold vacuum and maximum vacuum advance.
If you hold the throttle wide open and wind up the engine to several thosand RPM, vacuum will be coming up with RPM. Guess what would happen if you suddenly release the throttle to idle position? a HUGE vacuum spike. This is hard on Vacuum advance cans (if you tend to replace them often, check your vacuum connection). However, the ported vacuum saves the day by blocking the spike from the advance can.
Now, all this is for systemd DESIGNED to work with ported vacuum.
YMMV
