Well, if I'm a member and my friend's a member, and he's over here at my house and wants to post something under his name real quick, then a Log Out option is necessary so he can sign on and post under his own name, right?
The potential is there. My wife, Stephanie, keeps swearing that she will sign onto HTF with her own account so that she can post separately from me (every blue moon, usually over at After Hours, she'll post under my name...never anything not representative of me, though; but then again this boxes her in, so why not let her post her own thoughts under her own name?).
The fact is that noone has to use the logout feature who doesn't want to, but for the people who do (even if it's just a few of us), it's available. It doesn't hurt anything being there, does it?