I used to use a lock screen for fear of the dreaded Butt Dial.
(True story: I was in the audience for a play -- a ghost story -- and I put my phone on vibrate and turned it off, before placing it in my pocket and ignoring it. At the end of the play, the phone started vibrating in my pocket. Scared me half to death.)
I now use a pattern, with which I have a love/hate relationship. Love, because OMG, there is so much sensitive data on my phone. I mean, sure, I didn't let it remember passwords for my alarm system or my banking, but I've got apps for both those things in there. And anyone who may get access to my phone would immediately know where I live (thank you "my home" in Navigation), could tweet and email as me, could call my parents and claim to know me (for some relatively sophisticated fraud, if they were into that sort of thing), could buy stuff on amazon, and, for giggles, could mess with my thermostat.
OTOH, the damn thing drives me crazy because the phone locks when I'd rather it didn't -- like when it's in the car cradle. Look, if I'm going to do something of questionable legality like set my nav when I'm stopped at a red light, I want to do it with the fewest keystrokes possible. (Indeed, unlocking the phone while in the car cradle is why I switched from PIN to pattern -- easier to swipe a pattern than hit precise keys.) I would consider setting tasker to turn off the lock when it's in the car cradle, but I don't like to think a thief could bypass what little security I have by just jamming the phone in a dock.