I have 2 Ideals with the retractable sling and a Darne that does not.
I like the sling as a (it's just neat) thing and also for the ability to dig around in a bag or eat an apple without putting the gun in the mud.
I like slings on shotguns but I also like Belgian guns that usually have swivels
For duck hunting in saltwater a sling is mandatory as I don't want the gun in the mud ever.