I like the idea of carrying the deadliest primitive projectile weapon with me at all times.
I can sling pretty well and I am fairly certain I could hit a bird if I needed too. As far as a survival situation goes, I have gone 3 days without eating and it really wasnt that uncomfortable (after the first few hours). So throw in some wild edibles and a few doves and maybe a squirrel or rabbit and as long as I was just on my own, I'd be okay.
It's also quite entertaining to picture myself using the sling in different situations. Zombie apocoplypse, The Hunger Games, Wilderness self reliance, American Revolution, and Ancient Israelite war are just a few that come to mind.
That, and slinging is incredibly addicting. And the thought of being more accurate than a bow is quite attractive as well.