4. There is no easy way to close all the apps at once. Everyone I know has to manually go through each app and close them, just as you said.
5. No one has ever fully addressed the issue. I'd like to see some tech people (or even John Gruber) explain their take on the seemingly useless feature.
1. Most apps (as you said) don't support it
2. The apps that I switch back and forth between are not the ones that support it
3. There is no alternative to keeping the app open when you exit (see: http://www.azarask.in/blog/post/the-problem-with-home/)
4. There is no easy way to close all the apps at once. Everyone I know has to manually go through each app and close them, just as you said.
5. No one has ever fully addressed the issue. I'd like to see some tech people (or even John Gruber) explain their take on the seemingly useless feature.