Or you're running Windows. I've set up an internet connection for a family friend where only one computer was connected to the DSL modem but I configured it to use NAT anyway. This wasn't something I particularly liked, but I'm sure he's never experienced any drawbacks from it and no matter what he does with Windows, there's always going to be a little extra layer of protection.
To take this one step further, it would be extremely nice to have the OS and some commonly used files available on the NAND drive and another hard drive available inside the notebook. Most of the time I would be able to run with the hard drive unmounted and powered off, saving a nice amount of power, and when I needed access to those files OS X could spin up the hard drive and mount it with no problem.