There is a principle in bike fit that the handlebars should be as wide as the shoulders. Admittedly, this is more for road bars, but I have tried the same principle for MTB bars. I suppose that if your hips and shoulder width are the same, you've solved the issue. For me, 38cm is not such a big shift from the 42cm bullhorns that I have run on my fixie.
I find it a little amusing to see people riding flat-bar MTBs on the road with standard width handlebars that make them look like a goony bird coming in to land. They would do themselves an aerodynamic (and probably a physiological) favour by cutting the bars in a bit.
I quite like the look of narrow bars on the bikes we are talking about. It sets them apart. And I do ride quite a bit with hands on either side on the stem on road bars, so again, it would not be a great leap of faith for me to use narrow bars like those described... for certain, non-strenuous-climbing conditions, though.