There are two primary uses for a sling:
Providing stabilization for aimed fire.
Keeping your weapon on your body when your hands are busy.
I've used, in the .mil and busting the brush on the border, one-, two-, and three point slings.
My observations, purely my own:
A one-point sling is tacticool, and functionally worthless.
In the .mil, I had three two-point slings, all for the M-16 platform:
one for parade/inspections
one pre-adjusted for rifle range quals
one everyday to sling my weapon, drag in the dirt, etc.
Whenever possible and no one was looking/griping, I removed the everyday sling and simply carried the weapon, which proved ideal.
On the border, I fiddled with every permutation, for Mini-14s, M1A, and AR platforms, amongst others.
I have settled on a QD two-point sling, which I detach 99% of the time to carry my weapon with my paws, as God and Chesty Puller intended. Period.
I have never seen the day I couldn't hold even an empty or malfunctioning rifle, and draw a second weapon with my strong hand.
(Obviously, I haven't been shot in either arm, which is the only notable flaw there, but at that point, any sling is going to make fuck-all difference either, right?)
If I'm not stabilizing my prone position marksmanship, or climbing a cliff face on rappel, a sling is mainly a good way to get tangled in brush and brambles, trip you, or pound the butt into your nuts or the muzzle into your face without any concern for your welfare.
Other than as a basic transport aid, their main function seems to be to amuse your companions with your predicaments.
Take the damn things off unless you actually need them at the moment.
YMMV.

"There are four types of homicide: felonious, accidental, justifiable, and praiseworthy." -Ambrose Bierce, "The Devil's Dictionary"