I have the feeling that in the beginning, the idea of preserving your tabs was precisely because you wanted them to be prerendered. Then it became a way to have some sort of "front line bookmarking"
Tabs/history/bookmarks I believe are the most pressing area for UI development in the browser.
A lot of this management could be pushed into the OS shell. If you could manage Windows easier - you wouldn't need tabs.
Tab behaviour is so inconsistent across browsers, apps and different OSs it hurts.