Each of the examples you cited could be accomplished by enabling background streaming only. Stream last.fm or Pandora in the background and run the other app you need in the foreground. You can't have more than one foreground app up at a time anyway, and I can't think of a single app that needs to be run in the background that's not either streaming audio or messaging.* If you can think of one, I'd be happy to hear it.
* The possible exception are things like scrobbling or Google Latitude services, but such things would be such a drain on the battery running in the background that I don't think they will ever be allowed as third-party apps.
There are over 130,000 apps in the App Store. You can't just go by the usage examples that I've given. Someone many want three apps (or more) running in the background for a particular task. They may want Omnifocus always running in the background because it has a feature that alerts you of a potential task to be completed in the area you are in using GPS. There are tons of different usage examples and the only real answer for that is true multitasking.