I believe the maximum length to remain open in the background is 10 minutes. There are some apps and functions that simply need more time than that.
Also, take an IM app for example. When I receive an instant message in BeeJive or Whatsapp, the message isn't in the app when I open it even though it was pushed to my device in the popup. I still have to open the app, wait for the message to come through and THEN begin to respond (again, this is only when the app is force closed by the OS). It's stupid and counter intuitive. The app should be able to stay in an idle state in the background as to not waste battery life but still be able to wake up when a message arrives!
I don't think the maximum length is 10 minutes. I don't believe there's a limit. iTunes U (again, an app from the App Store) was still downloading video clips well after forty minutes I started (and closed the app) (I was downloading about 5 GB worth of video clips).
And I'm not too bothered by that last bit. I'd rather prefer to save up some extra battery life and wait 0,5 s when I open the app, than lose some battery life and start 0,5 s earlier.