Because the question was asked, I've been doing some research around the web. Also, because my bike came with a 170/50-17 and I've had handling issues when cornering (see below). So here's what i've found...
Choosing the right tire size for your motorcycle rims - Sport Rider
Most of the info I found was on sport bike sites. I guess they like the poser effect of the big rear wheel.
Basically, there is lots of ancedotal evidence from folks, on lots of different sites, who put wider than stock tires on the back. Most of them experienced negative handling characteristics. The general concensus was the bike was slightly harder to turn in which would quickly develop into a 'falling over' feeling. They claimed the harder to turn in felt like a tire that was worn flat in the middle from too much straight slab. The falling part... well, lets just say you have to experience it.
Most claimed that the general handling characteristics were worse than stock.
The other comment was that production engineers go to great lengths to match tire sizes with other bike geometry like rake, trail, swing arm angle, etc. to come up with good (and safe) handling characteristics. When you put a wider tire on, the entire geometry is messed up and handling in the corners suffers. And isn't that why we ride bikes? For the twisties?
This of course has nothing to do with darksiders.
I have a new set of tires (stock size) that will be going on my bike before the spring riding season. I'm tired of the feeling of the bike falling over in corners (and scaring the shite out of me) or pushing through them if I back off a little. I really struggled during the Lee Parks Advanced Rider Course. The Elka/Sonics suspension upgrades only helped moderately. I now believe I've found the culprit.
If you don't push the bike in corners and ride a lot of highway miles, a wider rear won't be noticed. Be careful if you like to corner hard.
My personal 2 cents and what I found on the web.