John Gruber, writing for Daring Fireball:
An awful lot of very hard work went into making iOS automatically manage background tasks like this. It's a huge technical advantage that iOS holds over Android. And every iPhone user in the world who habitually force quits background apps manually is wasting all of the effort that went into this while simultaneously wasting their own device's battery life and making everything slower for themselves.
Craig Federighi, senior vice president of software engineering at Apple, when asked if he force quit apps and if force quitting helped battery life, as quoted by MacRumors:
No and no.
Yours truly, way back in 2011, on iMore:
There will be rare -- rare -- occasions when a specific app, even an Apple app like Mail, stops working properly and a force-quit can get it to restart and behave itself. Once an a while your iPhone or iPad might get really sluggish and closing any big, recently played games might help.
But when it comes to closing ALL apps, ALL the time, just remember:
You don't ever -- never as in not ever -- have to close ALL the apps in your multitasking, fast app switcher dock. It's a sniper rifle, not a nuke. So just relax and enjoy your apps and let iOS do the heavy lifting for you.
And in 2012, on why Apple Genius' sometimes recommending force quitting apps, also on iMore,
If a customer comes to the Genius Bar with one poorly coded app or rogue process that's continuously slowing down their iPhone, iPod touch, or iPad, or causing massive battery drain, figuring out which app it is, and fixing it, can take a lot of time and effort. It can involve installing system monitoring tools, rebooting a lot, launching apps, testing, checking system status, killing apps, rebooting, deleting apps, reinstalling apps, etc. etc. It can involve a lot of things that some Apple Geniuses believe mainstream, non-technical users will have trouble understanding and doing.
It flies against Apple's recommendation, it flies in the face of best trouble-shooting practices, and it makes advanced users cringe, but...
Killing everything, in that specific case, for mainstream users, is the fastest, easiest path to problem resolution.
Personally, I almost never kill all the apps on my iPhone and iPad. I say "almost" because there are times when I'm running benchmarks or testing a beta or doing something else abnormal that I'll really need to it. Otherwise, I let iOS be iOS and manage its own resources and task running.
And, unless you're a software reviewer, developer, quality assurer, or someone else who really needs to be abnormal, so should you.
I do, however, force quit Facebook, Snapchat, and Pokémon Go far more often than I should need to. As far as I'm concerned, any app that goes out of its way to stay alive in the background, while draining my battery so fast it's almost visible in the indicator, has it coming.
So, my advice has always been and remains this:
The fast app switcher allows you to force quit apps for a reason. It also doesn't let you force quit all apps for a reason. If and when something goes wrong, use it with caution. Do it when you need to. Never do it just because you want to.
Because then you'll be the cause of bad behavior and excess battery drain.