One of the reasons to swap the back is to add durability. Covering a glass plate doesn't change the fact that it's glass. Swapping the plate is extremely simple and is actually quicker and then trying to apply a sticker straight. Very little can go wrong. About the worse you could do is lose a screw.
That said, the quality of the plates can vary and the beveled ones seam to be fundamental flawed by blocking the camera.
Exactly. Durability.
I'm still waiting for mine to come in from eBay. Should be here next week. Hopefully it doesn't negatively affect my reception.
Also, the metal back will look awesome with my Element Case. With the metal back the phone will be pretty solid feeling and still retain the same iPhone 4 look.
Besides, I haven't found a skin that looks identical to the iPhone 4 back. I don't like the invisible Shield style since I can never put those on right without fogging up.