Same. It reminds me of the "brassing" you see on well-used cameras that have a black finish over metal. The coating wears off in a distinctive way with heavy use.

Example: http://leicaphilia.com/wp-content/uploads/2014/05/BP-M3-5414...

I put an Apple case on my iPhone 6 when I got it and recently took it off and the side screen swipes work a lot better. It was clearly designed to be used without a case, so it's strange that they sell them. Oh yeah, they make 20,000% profit on each one :P

I just worry about scratches though because I always sell my phones when I'm done with them.

Unfortunately mine don't so much "wear down" as "fail catastrophically and suddenly."

I used to take that approach, then I shattered a Galaxy S, then a Galaxy S4 (twice!), a Sony Xperia Z3, and an iPhone 6. I now accept the bulk and hope it will preserve the contents of my credit card better.

If you plan to resell it (either as an upgrade program, or as used pre-owned) then a case helps you retain resale value.

Used to be I could just gift to my relatives, but they're pretty happy with their iPhone5's.

Agreed. The case for having a case is when you want to keep resale value as high as possible, but even now that's not such an issue if you do the upgrade program where you automatically get a new phone every year and basically lease it.

This, IMHO phones are not supposed to be jewelry.