While using other information might help, is there any reason an evil script won't be able to send your user agent, or any other information about your browser to it's author? So instead of just stealing your cookie, it'll report the user agent, and anything else required to fake a logged in session. The only thing that couldn't be faked is the IP address, but you can't do sessioning based on IPs, as often there isn't a one-to-one mapping between users and IPs.
That said, I think it's a bad idea to try to outsmart hackers - there's always going to be one smarter than you thought. Any reason why not to bad JS from the public forums?
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