Josh Ginter on apps that implement sloppy swiping through loose gestures that can be activated anywhere on the screen:
Stretching your thumbs to the topmost corner or having to swipe from off screen to go back is not natural in any way. Mobile phone users often find themselves in situations where the precision of pressing a specific button is both inefficient and aggravating. Swiping from off the screen can also be aggravating, especially when using the iPhone with your right hand when on the move. I actually find inaccuracies with swiping from off the screen to be more annoying than having to button-mash to go back a menu.
I'm a fan of the system-wide Back gesture introduced with iOS 7, but I prefer apps (like Unread and Facebook Paper) that don't force you to swipe exactly from the edge of the screen. I find that kind of gesture implementation comfortable, friendly, and natural.