Here's my logic. A pristine iPhone sells for a helluva lot more than one with dings and scratches and other cosmetic damage sustained over the year I usually own the phone. I rock the black Apple leather case on my 6+ which I don't think is ugly at all, as well as Tech Armor glass over the screen. When the 6S+ arrives, I shall pluck the 6+ from it's case, remove the glass protector, give it a little polish and sell that baby with a confident 10/10 quality rating. That's why I use a case.
As to the other guy asking about my underwear. *shrugs
Have you tested the premise that a pristine iPhone sells for a helluva lot more?
I did some quick ebaying a few years ago when I got my new iPhone5 and was worried about it loosing value. I looked at used iPhone4 and 4S models on ebay.
As far as I could tell, a pristine iPhone sold for only about 10% more than a scruffed up one. Indeed, there was almost no difference in price between having a few minor non-screen scuffs and mint condition. Prices only dropped more than 10% when it was very damages - cracked glass and all.
You paid ~$60 for a case and a screen protector to preserve about $60 in a value, the way I see it (odds are, when the 6S is released, a 64GB iP6+ won't be worth more than ~$600 give or take even in the best condition). I think you're going to break even only, give or take again.
That said, my iP6 has been naked since I got it in October. If I cleaned it out with some compressed air and a nice cloth, I don't think you'd know it I've kept it naked all this time. The iPhone6 so far has been much much more scratch-proof than the iPhone 5.
Add into the equation however much it is
subjectively worth it to you to use the iPhone unobstructed by gimmicky condoms. For me, it's worth something. For me, I'd say it's worth another 10% of the value to be able to use it in it's best cleanest form. So for me, a pristine iPhone should sell for 20% or more than a scuffed up one on ebay for it to be worth putting it into a case and screen protector.