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Yeah, I remember that. Definitely not great from a security standpoint, since once you've booted your own OS you can do nearly anything you want with the computer, like patch the firmware to automatically install a rootkit when the system boots.
Not terribly likely, but since a lot of public computers are in libraries and are used by people without home computers to do things like their taxes or online banking, a nasty compromise would hit a lot of people who wouldn't know their identities were stolen for quite a while.
The fix makes it a lot harder for computers to change operating systems dynamically like that, which is a bit of a loss, but ultimately worth it I think.

What I'm picturing would be a lot more like a static file format for sharing that information, without either device having the ability to actually "change" the other directly.