I don't think it's pointless simply because of the price point. You'd be hard-pressed to find a significantly better quality or sounding 2.1 soundbar for this price. Think of the 2 extra rear speakers as a bonus- you can choose to not even install them if you wish. A 5.1 source does not have sound coming through the rears at all time.. it's simply environmental- the most important channels are the 3 (or 2) up front.
Depends what you are looking for. The Vizio is not a premium audio product imo.. it's a consumer-focused product that sounds great to 95% of the population and can fill a room with better sound than what comes out of the TV. But for a PC set-up, you can buy something like the Logitech Z506 which is far cheaper than the soundbar and works just fine. If you don't need surround sound and value quality sound above all else, you can get a premium stereo set-up like AudioEngine A2+ on sale for around $200.