I have two Darnes equipped with the bretelle Darne. Another Darne has typical sling swivels, a good thing, as it is a heavy (heavy for a Darne, anyway) 12 gauge R10, with 28" barrels with mod and full chokes, that I speculate was laid out with at least consideration for waterfowling in Europe, when it was built. Sling swivels were the cheapest option for a sling on a Darne, and a lot of people just went with them. Last Darne 20 has no sling. Good for trips like grousemas, where Lloyd and I do a fair amount of driving from spot to spot.
All have a place.
I have a pump 12 that has a sling, but, it has quick detach swivels, so the sling pops right off. I've never waterfowled with it, but, friends have. I suspect waterfowling with a gun without a sling would be a lot more work than with a gun so equipped.
Two deer rifles, of a type usually considered for shorter range brush work, left to me from my Father's estate are graced with slings and his preferred Redfield 1.5-6 optics, in respect to his advancing years. I'm not a deer hunter, so the Marlin 30-30 and the Ruger .44 auto carbine haven't been fired in decades.
But, a sling makes them better, of this, I'm sure. Pretty sure that is the case with Stan's big Fox, when hunting ducks, also.
If a between-the-wars name English gun falls into my lap, I won't be looking for a sling for it. Not every gun needs one. But, some of the hunting I do is made better with a method to keep my hands free for a moment or two, and a sling allows for that.



An image just for jOe, 'cause he likes these guns so much.

Best,
Ted