Any heavy background processing will cost you battery life. If you don't do that, and you have only one foreground app, you have inherently parallel tasks handled (playing audio, uploading, downloading files, etc.), anything asynchronous with push notifications, you don't really need to keep the apps currently not on screen running, you can suspend them and conserve even whatever little power they would consume. If you can store/restore app state efficiently you won't see much of a delay when switching between apps.