From ‘Dive into HTML”:
“So we want unique resources to have unique URLs. But at the same time, browsers have always had a fundamental limitation: if you change the URL, even through script, it triggers a roundtrip to the remote web server and a full page refresh. This takes time and resources, and it seems especially wasteful when you are navigating to a page that is substantially similar to the current page. Everything on the new page gets downloaded, even the parts that are exactly the same as the current page. There is no way tell a browser to change the URL but only download half a page.”
Finally fixed in HTML 5 with History API.
“Good riddance to bad rubbish.”