Apple's strategy is to not burden their users with the mental load of having to make that tradeoff. Can you imagine trying to use this device for a whole workday and having to worry about whether you quit that last app so that the battery will last until your shift's up?

For casual multi-tasking, the speed of the flash memory means that well-written apps should be able to save and restore their state pretty quickly even if the operating system does quit the app when the user switches to a new one. More serious multi-tasking (where you would want side-by-side windows and other major UI changes) would definitely reduce battery life and make things much more complex.