Yeah Rev, Bengt[piper] is right. It's definitely a "look" thing, as in most cases mounting a wider rear tire than is recomended by the factory usually degrades the handling of almost any motorcycle, due primarily to the tendency to "plow" into turns with that "too wide" rear tire.
In other words, the wider rear tire will tend to put more straight ahead force onto the bike in turns and somewhat overstress the front tire's contact patch.(once again, otherwise known as "plowing" in turns)
Case-in-point: Almost all motojournalists who have ridden both the Triumph RocketIII standard with its 240/50-16 rear tire setup and the Triumph RocketIII Touring with its 180/70-16 rear tire for their magazine evaluations have made it a point to mention that the latter bike mentioned handles much better in turns, due primarily to its narrower rear tire.
This "wide rear tire is cool syndrome"(as I call it) is somewhat akin to what many folks do to their rear suspension when they lower it. I suppose it "looks cooler", but when folks do that to their bikes, they tend to degrade the handling abilities of their motorcycles also, both in ground clearance and in weight-distribution, resulting in less weight upon their front tire, which then makes the steering lighter and giving less feedback to the rider.
(hope that answers your question)
