[rottencupcakes]

Because rubber feet have friction and hold a phone on a surface much better than the iPhone's smooth back does.

Also, putting my iPhone down without scratching the front or the back glass on a piece of sand or dirt is an art.

Yes, the iPhone 4(s) is pretty, but you sacrifice function for that beauty.

[tptacek]

If you want little rubber feets on the back of your iPhone, put little rubber feets on the back of your iPhone. The back of the iPhone is, properly, inert and ambivalent to your modifications.

[ceejayoz]

Agreed. Thus far, every time my iPhone 4 has fallen has been a time I put it on an ever-so-slightly tilted surface and it seemingly slides under its own power. I'd be fine with a little more friction on the glass backing.