The Digital Darwinism Point of View
Clay Shirky, Professor of New Media at Hunter College, has written an open letter to Jakob Nielson to put forward his opposing view of usability design. His view is that, since the web is a dynamic, evolving system it is already improving from the usability point of view. He's probably right: if usability has anything at all to do with the popularity of a site then the overall usability of the web will improve naturally as people seek to emulate the successful sites.
If only the top 1% most trafficked Web sites make usability a priority, those sites would nevertheless account for 70% of all Web traffic. You will recognize this as your own conclusion, of course, as you have suggested on UseIt (http://www.useit.com/alertbox/9704b.html) that Web site traffic is roughly a Zipf distribution, where the thousandth most popular site only sees 1/1000th the traffic of the most popular site. This in turn means that a very small percentage of the Web gets the lion's share of the traffic. If you are right, then you do not need good design on 70% of the web sites to cover 70% of user traffic, you only need good design on the top 1% of web sites to reach 70% of the traffic.