Just a screen protector. The back of my nexus 4 has a crack in the glass though. I am willing to take a chance to get the form I want out of my phone. I hate going to shop for a phone and liking the size and feel, only to throw on a case and screw it up.
Have a Galaxy in a case. Its an Otter rubber case. It protects the glass also i think. Its been mishandled and dropped on the cement by the grand kids. (twice). No issues.
Oh well........
I've used several high quality cases, but forewent the screen protector (no point IMO). I had a Speck, Seidio Active, and currently use an Otterbox Commuter for my Inspire 4G.
I don't have my Droid 2 in a case; it's pretty durable as-is since it has a rubber-ish back and a pretty sizable ridge around the screen for face-on drops. Also, it's already too thick to begin with.
I think it is essential for touchscreen phones as they easily get damaged. I am one clumsy person and I know that it would cost me more if I won't put a case on my phone. Better be safe than sorry.
I have just a screen protector on my Droid Razr. I've tried different cases in the past but they were always too bulky or a PITA in some other way so I stopped using them.