I think people that use the term "investment" for their phones are just using the term incorrectly.
I try to keep my phone nice, first and foremost, because I like it looking nice. I don't walk around with mustard stains on my shirt, holes in my shoes, and the hood of my car crumpled from last week's fender bender. Secondly, when I either pass on or sell the device, I don't want it to look like a pile of junk. Lately I have been giving my family devices, and I wouldn't give them a scratched up pos. a few years ago I would sell my devices. Fact is, people are willing to spend more on a device in good shape. Some of the banged up phones I have seen I wouldn't pay half of what the current market asking price is. So is it an investment? No. But it could be the $20, $50, or even $100 difference if/when you go to sell the thing.