What is the point of buying a metal phone and placing it in a plastic case to prevent scratches? Just buy a phone with a plastic shell. A good chunk of the purchase price goes towards making these smartphones thin and looking good. Adding a case makes them simply thicker and uglier than a caseless phone with a few scratches here and there.
I don't want the phone itself to have scratches on it. That's not an unusual feeling.
One reason? Resale value. My iPhone 4S has been in a case for the last 9 months... and it's still in pristine condition.
Glass or metal phones will pick up scratches from any fine grit they may come in contact with on surfaces. That's something I would like to avoid.
The nice thing about other plastic phones is that you can easily replace the plastic back or battery cover if you need to resell it.
As for thickness... my iPhone 4S with a case is still thinner than my old Blackberry without a case.
And an iPhone 5 with a case would be thinner still.