Think of climbing as heavy lifting. Why hold your arms out from your sides when you lift?
Strange comparison there. Weight lifters will use a wide grip for lifting heavier weights. Ever seen anyone doing squats with their feet close together? Same goes for bench press, a wider grip is better then a narrow one.
Anyway, the ideas is that everyone should use the width that is better for them, and as someone mentioned above the way to find it out is to try different width handlebars.
As was pointed out in that post, the only reason for such wide stance was extraneous to the real issue of lifting. For squats you use a wide foot stance and your arms have to be outside of your arms for obvious reasons. For deadlifts, arms still have to be outside, but only because of the obvious reasons again. Your greatest lifting ability is parallel to your stance.
Lifting is cited because if one is competing seriously at a high level in climbing, one has to put out significant power and how your whole body comes into play is important to how much power you can put into a lift or into climbing a steep incline at speed. Climbing, though, can be a longer aerobic activity. It's simply what a poster commented on and deserved a reply. Bar width isn't as important in climbing as it is, say, in the finish to a big road sprint where the positioning, power transfer, and aerodyamics are all critical to final performance.
Most significantly, it's about getting out and racing different equipment. We're each an experiment, and comments like this one about narrow bars are good guidelines for most, but not necessarily for all. And these comments are about how to race. If you want to put on your Lightweights and go out for an 18 mph cruise along the Pacific Coast Highway, it really doesn't matter what bar widths you use. People on is forum look to the best racers to see what equipment they race with; it's also appropriate to look at how
they race as well. But again, if you aren't trying to achieve the speed, then don't worry about it and just enjoy the ride.