The point to multitasking in iOS 4 is pretty straightforward to me...
Some applications like Pandora and Evernote run in the background only when playing music or uploading files. When they are not doing either they are in a frozen state, not consuming any resources or battery life.
So you don't need to close any of the frozen state applications, ever. My battery life has not decreased at all with this new update, nor has performance.