"Extra"? Heh...wish I had that kind of hardware just laying about.
Well, the pro would be that you already have the gear. Con is that it's not quite as expandable if you wanted to add a PCI card for something, though most everything these days can be found in a USB device. Also, finding something with the appropriate power output can be tough, and managing a shutdown circuit with a laptop could also be hard without popping open the case and doing a bit of soldering.
But once again, having just about all (or all if you intend to mount the laptop up front and use its keyboard/touchpad/display) really cuts down on the cost of implimentation.