Really, handlebars don't work without anchor points, in most cases, this means your hands (stunts excepted)! So, if you ride in the drops all the time (like track racing), then you are riding the advertised width of the bar. However, if you change your hands to the tops, as often seen on the road, you are reducing the space between your hands, and effectively riding much narrower bars.
The counter-argument to the idea that wide bars are better for opening the chest cavity and therefore should match shoulder width, is that in a intensive cycling position your bent elbows do that naturally anyway -- witness the number of pro climbers that will ascend on the tops (effectively narrow bars), elbows out; or sprinters' protruding elbows in a sprint. Although I don't win sprints, so there might be a lesson there.
For cornering at speed on the road, you will be countersteering, which reduces the amount of leverage needed to turn on most bikes. Wheelflop might be an argument for wider bars, but then do people riding identical bikes with 40cm bars turn less efficiently than ones on 46cm bars? I dunno.
Can you guess I ride 38cm road bars yet? They are skinnier than my shoulders, but I find them a lot more comfortable than wider bars, but still provide enough hand positions and control. I know Adam Hansen rides 38cm bars, but after seeing his, er, unique bike setup in close range at the TDU, I'd wager his reasons might be different to mine. But then again, 42cm to 38cm is what, 4cm or roughly 10%? Might be more psychological than physical...
I'd ride narrower on the track if I could find them cheap enough, hint, hint.
