The vacuum advance, as noted, only affects part and light throttle driving. It is designed to maximize fuel mileage and keep plugs cleaner over their life time. Properly set, it does not negatively affect the engine at wide open throttle or idle, and in fact can add 10-30% more mpg depending on the setup and tune. It is a feelable difference when you're driving and I always run them when possible. There is a point where the vacuum characteristics of an engine package make it impossible to adjust the advance to work with it. That's usually with larger cams or engines that are overcammed. At that point, it's better to cap the line and run without it.
