I jump on Facebook. Type a clever comment on one of my friend's walls. Press the enter button. No! I was logged in as my wife! Now, she'll get the credit for my hilarity!
This happens in our household all the time. Having to log-in and log-out of Twitter, Facebook, iGoogle, Hotmail. It is SO annoying. And there was no super easy way around it. So, since the browsers/sites didn't have an easy solution, we made our own.
I use Google Chrome. Kat uses Firefox. We're always logged in. No more mistakes. No more wasted time.