Sounds like your friends need their own damn phones or maybe a computer...
They have regular old phones. Some of them have smartphones but only have WiFi. This is usually when we go out or something. You know, waiting for a movie to start, eating at a resturaunt. I'm not saying they do it all the time, but a prank call to someone I know on MY phone? Once is already once too many.
Another minor reason. Sometimes I'll let my dad use my phone for the navigation. He then proceeds to go through all my texts and other private information like call history. Texts and call history I can delete, but not my contact list.