Wider handlebars have slower steering than narrower bars. In other words, to turn the front wheel a given number of degrees, a rider's hands have to move further forward or backward on wider bars than on narrower bars. That is because the lever arm is longer on wider bars.

Wider bars give more leverage to turn the front wheel than narrower bars. That is also because the lever arm is longer on wider bars. This may be much more important in off road riding than in on road riding.

Narrower bars are more aerodynamic than wider bars, but in the real world, this may or may not make any appreciable difference. It depends on the rider and the circumstances.

Narrower bars can navigate through smaller openings than wider bars, however the difference is only a few centimeters. For a bike messenger in New York City or a racer in a peloton that might make all the difference. For most everyone else, that might not matter at all.

Being able to breathe better with wider bars than with narrower bars is very much open to debate. Time trial riders use a tremendous amount of oxygen during their competitions, yet they have no problem breathing with their arms close together and their hands right next to each other.

Some people like wider bars, and other people like narrower bars. This may be due to the rider liking quicker or slower steering, because the rider grew up with a particular type of bar, because of the popularity of one type of bar over the other, because one type of bar "feels better" to the rider, because the rider has heard that one type of bar is better than the other, or because of any number of other valid and invalid reasons.

In a world of hyperbole, it is important to keep one's head and realize that one size does not, in fact, fit all. And, the worth of wider or narrower handlebars needs to be determined on an individual basis.