I keep my phone in a case - in fact, I have three, because I didn't get on much with the other two. Just to protect the screen and stuff when it's in my pocket, and otherwise, it doesn't look like a screen which will scratch easily, but it's the method of controlling the phone, and I don't want it to get damaged, and screen protectors are a right pain in the neck.
It's common sense, it's an expensive item and you want it to last so you look after it, I'm the same with my laptop, and you get surprised how roughly people treat theirs (mainly kids, it has to be said) and then moan when the casing cracks/USB port comes out/power jack snaps when a bit of safer handling would stop it - that said my laptop did escape remarkably unscathed from a somewhat closer encounter with some red french stuff than you'd imagine, the other week.