This subject comes up every few months

Agree there not totally un-cool, but in the right circumstances.

If you have an older MTB, with 540/560mm handlebars, they are very useful for additional leverage, but on a newer bike, with a minimum of 680mm bars, there have limited to no use, as the extra leverage isn't needed, and there are the issues of catching trailside objects as already mentioned.

Would look at how many people you see riding with barend today, suspect very few, if any unless they are riding older bikes.

There are alternatives now available , like Togs, which are more for the endurance (not Enduro) or XC riders, both now relatively niche markets.