I hate "just kidding" in any way or form. It basically means:
I completley meant what I said, but I don't want my ass to be pounded for saying it. Example:
"wow your a f@g jk"
"dude tht sucks jk lol"
"i r way more pwn than u jk"
People in my classes and on the internet use it all the time. If you have something to say, say it and don't be such a puss about it.