I like wide bars on bikes, whether they are on a mountain bike or a road bike. I rode with 52cm road bars for a long time, and I especially liked them on cyclocross bikes. But, there was a time when I ran narrow bars on my mountain bike, despite my preferences.
When I got my first Cannondale, in Ohio, it had a nice wide set of bars on it. The idea was that the wide bars allowed you to customize the fit, by cutting them down to your preferred width. I preferred the widest bars possible, so I left them at full length. I did move the grips and controls in a bit, on both sides, out of necessity when I installed Onza bar-ends. But other than that, everything stayed as it came from the factory.
I signed up for my first race not too long after getting the Cannondale. It took place in a state park, southeast of Columbus, on trails I had never ridden. I looked forward to riding new trails as much as I did the race, itself.
The day of the race dawned cold and wet (it was early Spring), and the trails were pretty muddy. The race looked to be a mudfest, and it was. Midway through the race, I was getting a bit tired of climbing hills in the slick conditions, and I was glad when the trail turned downhill toward a creek crossing.
The trail was still sticky, wet clay, but the slope was steep enough that I was able to get up a pretty good head of steam. I was going somewhat over 20 miles per hour when, as I dodged a tree to my left, I hooked my bar-end on the trunk of a tree to the right.
I had never realized, up until then, that trees know Kung Fu.
After picking myself up out of the mud, I still wasn't entirely sure what had happened. I looked back at my bike, still hooked to the tree, and figured it out.
Not too long after that day, I bought a Scott AT-4 handlebar, in the smallest width available.. The AT-4 was formed in a closed loop (a plastic bridge connected the ends, at the front), which provided a number of hand positions and also acted as a knuckle-guard. You might hit the bar on a tree, but it couldn't hook the trunk.
That bar treated me well in the tight, tree-lined singletrack of Ohio, but it proved a bit squirrelly on the long, wide-open downhills along the Front Range of Colorado.
Nowadays, I run my bars wide, because the trails where I ride allow it. But, I am still nervous when I pass between two trees, on either side of the trail.