The worry about back glass of the iPhone 4 cracking applies only to those cases that slides and covers the iPhone 4 back in entirety. For example the Grovemade and Incase Sliders. Reason being, grains of sands can enter and get trapped between the iPhone and the case, rubs into the glass and cause microscopic ridges and cracks that further exacerbates into the glass back cracking.

With that in mind, would Apple revise and realise the importance of screen protectors, especially the back part? As for my current configuration, the Vapor does not exhibit this worry since it never slides into the back of the iPhone. Additionally, I also have the PowerSupport screen protector for the back, and on top of that, the SGP Carbon Fiber. So, no worry of cracking back glass iPhones for me.