I can't disagree with any of your use-cases – but I think the clever app switching plus a few extras (remembering app state etc) would satisfy a whole lot of folks. Some power users will definitely agree on the 'not enough' side though. That's why I'd love to see a toggle, with whatever appropriate warnings Apple wants to give, that lets you toggle on 'full mulittasking' or similar.

Oh – I also like th ability in QuickDo and ProSwitcher to stop some of the built-in apps from running in the background.

Music apps — Mike11 asked instead for an API that allows music streams to continue in the background. This is a possible solution, but since there are many other use-cases for true multi-tasking, this would probably be more trouble than it's worth to implement.

Navigation apps — One of the main uses I have for multi-tasking is to keep Navigon running (and actively updating) in the background while I'm doing other things. I just recently took a long road trip to Atlanta. I was talking on the phone when I came up to a turn I needed to make. Navigon politely announced that my turn was coming up while I was jabbering away on the phone. No interruption to the phone call or my driving. As I checked Google Maps for traffic conditions, replied to IMs in Beejive (while stuck in non-moving traffic. I'm a safe driver, I swear!), and performed other tasks, I was able to keep Navigon running and updating and never miss a beat.

Games — Although most games wouldn't need active multi-tasking ability and a clever app-switching / state-saving system would suffice, there are some that still benefit from true multi-tasking. Games that are in constant communication with a host server to send and receive updates on the players location and other factors would not be able to handle a flash-freeze of their current state then a total shutdown. If the server stops being able to communicate with the client, it will consider it gone. There are also other games where it may be another users turn. While waiting for the other user to make their move, you switch to check your email or Facebook. With full multi-tasking, you will still hear sounds from the app so if there is a sound that lets you know you need to be back in the game, you'll know immediately.

I'm sure there are other use cases that can't be solved with push and/or clever app-switching. This is what I've come up with while still half asleep. The point is, multi-tasking in this day and age is something that is very important. Yes, there are better solutions in many cases. But I don't believe there are better solutions for all cases.

Actually, one of the things I miss most about being jailbroken was the ability via mQuickDo to quit the default apps that run in the background (like Safari). Saved on battery drain.