The iPhone needs curved sides to make it easier to hold.
How do you hold your iPhone? If you're like me, most of the weight is supported by your pinky because the sides are too smooth.
This has bothered me for years, and it's become an even bigger problem now that I have the larger iPhone 6 Plus. The longer you hold it, the more you strain your pinky, and it can even put strain on your wrist and lower arm.
The simple solution is to put a slight curve in the case, so it doesn't slide down as you hold it.