an AV is never NEEDED as the original question posed
In the purest sense of that statement you are completely correct. With enough time and knowledge a single computer or a network could be set up so that a virus would not be able to function on the system making the need for AV moot.

Taking the point above the 'need' or 'requirement' for AV is a result of the way the system was set up and administrated rather than an inherant 'need' of the system itself.

Even on a system set up to the standard you describe where AV is not 'needed' it could well be 'required' for insurance cover and to meet other regulations which may state a requirement for AV to be present on a system.
This is not to mention the comfort factor of having AV ,"just in case", as mentioned before.