The backing plate is necessary unless you only want to use the white pad that comes with the PC. Meguiar's and some other manufacturers have kits that come with pads and a backing plate and can be had for about $35 or so. You can use a wide variety of pads with one backing plate.
Extended version: The pad which comes with the PC sucks. It breaks down very quickly. Having a hook and loop backing plate will allow you to use whatever pad from whoever you want. Those backed polishing pads are horribly expensive. Why pay for a backing plate each time? And hook and loop is easy on easy off. It'll let you change from cutting to polishing to finishing pads in seconds. Also allow you to quickly change to a clean pad once the current one is loaded up. The backing plate is what makes the PC so versatile. Without it, it's very cumbersome to use.
Make sure you don't get the Meguiar's backing plate. Lot's of people, including me, have had problems with them not sticking after a while. Get the Coastal tool contour backing plate, or the Lake Country backing plate.
The only reason I didn't want to use a backing plate is because a PC take a big enough chunk of change out of my pocket, and I can't afford a backing plate.
If you dont have a backing plate, what else are you gonna use for your pads? If you order it from costal tool, they include one anyways, and a 6" counter weight, for $120 i believe...