How important is the rack you use for your components

As @newbee points out the effect is system dependent. In a more resolving system the impact may be substantial. For example in my system each step in moving my VTL power amps from spiked Grand Prix Audio Monaco stands to Apex (ball) footers and then adding a Marigo sub shelf under the amps was very noticeable.

The general effect of reducing vibration on an amp is to make the amp sound less loud (ie for a given setting of the volume control you hear less harshness and hence can afford to dial it up a few notches) and give greater control over dynamic peaks and hard to resolve passages. Impact on source components can be similar but more diverse. Turntables are another story entirely and need their own isolation solutions.

@randy-11 -- nice, but requires a pump and air supply to operate. Also I worry that they require a fairly hefty minimum load. 

@ricred1 

My question is if you have Isoacoustics , Stillpoints, or any other "effective" footer under all of your components, do you need a very expensive rack? If so, why?

In my experience the answer is yes. Think of it in terms of layers of defense. The footer is the last layer to isolate the component from external influences, but it stands to reason that if you reduce the vibration reaching the footer the less it will have to do, and the more effective it can be. In my case my source components sit on the following (from the floor up): GPA Apex footers (a roller ball system) -> GPA Monaco shelving system -> GPA Shelf -> Marigo custom sub-shelf -> Marigo Mystery Feet. So 5 different layers the removal of any one of which is audible

Arguably a solid wooden racking system will be the most in need of replacement as it may store energy and give the footers more to work with

There are of course exceptions -- if you are using an active isolation system such as a Herzan you want to firmly couple it to the floor and not have intervening layers of potentially competing isolation