I have never left my iPad lying anywhere, unless it is with friends, and I am not too worried about them getting a peek at my home screen, or my email...
It's not a binary decision.
Your e-mail is the key to your entire online (offline too?) identity.
I trust my friends but I wouldn't give all of them "me-level" access to my entire life.
I have never left my iPad lying anywhere but the bathroom. (That's the only place I find it actually useful.)