It is indeed andy that you can turn your Mac into a external firewire harddrive to save your data, but bad thing about this is that anyone else could also get to the data this way if he/she has a Mac and a firewire cable
Just like anyone can copy any file from your PC by booting from a bootable USB thumbdrive or Linux Live-CD, after which every file on the harddrive will be readable & ready for copying.
Once people have physical access to a computer, there is no security. (barring any full-disk encryption, which noone uses anyway)