All of my experience with slinging rifles came while in uniform. I own a few modern sporting rifles, but have never attached a sling to any of them because they live in my safe and are not my go-to defensive firearms. I fire the MSR's with my sons fairly regularly on the in-law's land, but have never bothered to add slings. Now that the world is going crazy, I may have to reconsider.
I like a sling to wrap around my elbow while firing standing up for the support/retention element allowing a more steady fire platform.
I would always keep a sling on a HD rifle/shotgun. What if you need to use your hands for something else?
It seems to me that the ready positions are primarily designed for M16/M4/AR15 rifles. Certainly they aren't designed for typical slings you would have on your hunting rifle. I probably wouldn't want to fire a 30-06 or even a shotgun from retention too often.
I could see high ready as a viable alternative to retention, indoors in tight spaces.
I can definitely see the advantage of the sling on a home defense rifle or shotgun, but I think my gut reaction when I sense danger would be to grab a handgun. Two of my handguns, my EDC and my nightstand gun, are more easily accessible from my bed than my shotgun (the rest are locked in the safe). If I am awake (and clothed) at the time a threat presents itself, I will most likely already have a handgun on my person. At this point in my life, I have had so much more training and practice with my handguns than my long guns that I think I would be more effective and comfortable with a handgun. Once .223/5.56 ammo or buckshot comes down in price, or even becomes available, I may finally get around to getting some much needed defensive rifle/shotgun training.