Fast user switching a security risk? Nope.
Well, look at it this way: It's a feature that's a heaven-send for families. Sure, the 14-year-old boy with his hacking-skills will read the 16-year-old sister's e-mail. But the parents won't know. ;-)
Well: You can password protect it. And that's as safe as really logging out. So no: It's not really a security issue.