Swiping away all those apps on your iPhone so they're not running in the background is a waste of time, apparently. Closing apps via multitasking doesn't actually affect battery life on the iPhone says Apple.
The SVP of Apple, Craig Federighi, told a 9to5mac reader that multitasking app on iOS doesn't affect battery performance.
When asked over email, "Do you quit your iOS multitasking frequently and is this necessary for battery life?", the simple answer given was: "No and no."
Supporting this unequivocally clear answer, Apple's own support pages do say that force-quitting should only be used on a case-by-case basis when an app freezes or misbehaves.
Since quitting all your apps doesn't save battery life that might explain why Apple has never implemented a close all option when in multitasking. Apparently despite being seen in multitasking the apps aren't actually running so won't be using any RAM.
Apps that use battery in the background are ones that are still in use, like navigation using GPS or music playing from iTunes, say.
So if you've got any other burning Apple questions it might just pay to try emailing the big dogs directly. You never know you might get an insightful reply.