Craig Federighi replied to a customer’s email:
Do you quit your iOS multitasking apps frequently and is this necessary for battery life?
No and No.
Kyle Richter of MartianCraft elaborated a couple of weeks ago on why this is completely unnecessary:
Many people think that force-quitting these apps will at the very least do no harm since “they aren’t running anyways.” The logic of “…you might as well quit, just in case” comes into play. The problem is that force-quitting apps that are Suspended, and not taxing the battery, produces negative effect and can do quite the opposite of the intention.
If you force-quit an app, it’s removed from memory, its state is instead saved to disk, and the app is closed or quit. This event triggers a multitude of tasks from disk i/o, to memory swaps, and even cpu cycles processing data. If the app is relaunched, additional resources are required to open it from a closed state as opposed to the faster Suspended state. Since the OS manages purging apps when memory is already low there is no benefit to force-quitting suspended apps, unless of course they are misbehaving and need to be relaunched.
Let us put an end to the misinformation right now.
In the overwhelming majority of circumstances, there is simply no need to manually manage applications on your iOS device. In fact, it’s almost always wasteful to do so; it is to your benefit to not force quit apps.