The Norton ( McAfee, etc.) provides reliable updates. It provides an unmatched support. It scans every file that you deal with. Moreover it has got its benchmark in the AntiVirus business. This means for a sustantially serious work going on in the field, these products are a prerequisite.
I agree that an individual can get another AVPs available freely, he can get a firewall, filters, etc, but he can't get that peace of mind as he would by purchasing these products.
Moreover an average guy doing his job on the Comp. does not bother ablout the types of viruses rendering in the background. For him its just another mosquito repeller, which is supposed to prevent his malaria.