Heard from a friend this morning in an email at about 3 a.m. - 

"I just realized a short time ago that I probably left my I-phone at Jordan Creek....  I know I was using it about 7 p.m. when I wrote a message to you.  Can't find it and I  can't sleep.  Of course, I did not set up the Locate My Phone function!"

Luckily she found it back home, but it goes without saying that there's no sense in having a Find my iPhone function if it's not used.  If you have it, you can send a message to the phone [a reward to the finder?], you can send a loud beeping noise to help you find it if you did leave it in your home, you can wipe the contents if it is out of your control, you can track it and let the Police know where to find the thief...but if you forget to set up the security, you're S.O.L.

So do it now!