There used to be a great write-up on the Davis Unified Ignition site about vacuum advance but it appears to be gone now.. The bottom line being there are more advantages than disadvantages.

Here is what it currently says:

Q: What’s the difference between mechanical and vacuum advance?
A: Mechanical advance is the centrifugal weight and spring assembly located underneath the rotor that provides the largest part of the
timing curve. The vacuum advance canister, located on the side of the distributor, provides additional timing only when the engine is pulling
vacuum. The mechanical advance along with the initial advance (base timing set at idle) is what gives you total timing. An example of total
timing would be 12° initial plus 24° of mechanical which equals to 36° total. Vacuum advance is not figured into total timing because it will
only function when the engine is not under a load. Example: If your vehicle is cruising on a flat stretch of road or going downhill, vacuum
advance will come in and add as much as 15° to the total timing for increased fuel mileage and cleaner spark plugs. It is not uncommon to
see as much as 50° – 52° of timing with vacuum advance. But don’t be alarmed by this being too much timing as the vacuum advance will
decrease once you accelerate and put the engine under a load. Under hard acceleration the timing will go back to the original total with no
vacuum advance.