Some might argue that the iOS' "one-way-to-do-things" design guidelines are constraining and limit innovation, but in practice they are very well thought out.
Double clicking it pops up the multitasker.
It isn't that hard to accidentally double-click the home button and get to the other running Apps. This is a natural effect that good UI depends on. Rather than filling the screen with a control for every possible option, you get this "hidden" functionality that users discover by mistake.
Once there, swiping left and right is an obvious thing to do, but even if you don't think about it, you again can do it by mistake.
I found the voice command functionality by mistake when I held the home button down while thinking about what I wanted to do next (And have occasionally accidentally invoked it in this way since.)
Still, one of the things I experienced in best buy was that the buttons at the bottom of the screen were pretty confusing.
The home button is really intuitive in comparison.
I hear complaints like this a lot, then try to replicate them on hardware I have to hand, and shockingly I am unable to do so! It's almost as if these stories are massively embellished versions of real interactions.
For example. Have you actually seen how complex the home button on an iPhone is?