[r00fus]

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.

[efraim]

So keeping it in a case for two years is worth the extra $20 when it's time to sell it?

[r00fus]

Spread between "like new" and "good" condition can be much more than $20, esp. on eBay.

[snuxoll]

I don't know about the iPhone Upgrade Program, but T-Mobile's JUMP plan doesn't give a crap about the condition of the body of the phone as it is usually replaced when refurbished for sale as a pre-owned device or use as a Like-New Replacement. As long as it powers on, doesn't have a cracked screen and has no water damage they'll happily take it (replacing the body is cheap in most cases, so why care).