[mr0077]

And to add to Ernie's post, under some circumstances (e.g., warm to radical cam), the idle vacuum may vary so much that if you use unported vacuum for the vac advance, it will cause the timing to "hunt" with the varying vacuum signal, changing the idle speed, making for an unstable idle and a real handful for tuning purposes.

If I remember correctly, pre-emissions cars (circa 1950's) had unported vacuum advance, and ported advance was used from the late 60's on to improve emissions, and is not a bad thing for most applications.