I have never cared for them but I find most of the field guns I purchase have them. I have found they mar the stock and the barrels so I remove them and place them in storage. The stock hole is filled with a brass engraving oval and the barrel has shot peened into the holes. The shot can be removed for later return of the sling swivel and the butt can be redrilled and swivel installed if the next owner wants it. It works for me , but I must admit I have never given the value a great deal of thought. I wouldn't make a very good collector.
Best,
Ron