Wide bars are ace. I pinched the 780mm bars off my DH bike to try for XC, thinking they’d be a bit stupid but I loved it, never went back. The only time they felt a bit wrong was technical climbs out of the saddle, it’s slightly harder to support your weight over the bars when they’re so wide. I’ve since gone down to a marginally more sensible 750mm Easton Havoc (Carbon 8) ) and I don’t seem to get this downside.
Like everything there is a limit to the “wider is better” argument, and of course it’ll be different for everyone. However, for people that haven’t tried anything beyond a 685mm bar, I’d strongly recommend at least trying something wider for a few rides.