I've served and trained with the "hard men" in a previous life, so I think I can contribute something to this discussion.
First, the point of a sling is to have the weapon immediately available to you when you need it. If you've got it slung (whether 3-point, single-point, or whatever) it's ready to go when you are. Having a weapon leaning against a wall, or a tree-trunk, or lying on the ground, is not nearly as accessible, and also exposes you to the risk of having to move suddenly (as Clint Smith puts it, "incoming fire has the right of way"!
) without having time or opportunity to grab it. Congratulations - you've now disarmed yourself of your primary weapon.
Second, if you're trained and have even a bit of experience, it's rather difficult for someone to take your slung weapon away from you. There are all sorts of counters and other weapons available to you. If someone grabs my slung weapon and tries to take it away from me, or use it to throw me off-balance, by the time he gets going, I'll have a handgun, or a knife, or some other weapon in one or both hands, and I'll not hesitate to use it to indicate my displeasure to him. (Don't forget that since his hands are on my slung weapon, by definition, parts of him are within easy reach of grievous bodily harm.)
Thirdly, a major point in CQB issues is not to make weapons available to your enemies. If you leave a weapon lying around, or leaning on something, it's not secured, and can be grabbed and used against you (particularly in a situation such as my first point above). You never, NEVER want to arm your opponent: and with a slung weapon, you won't, unless you're badly hurt or dead, and can't stop him unslinging it. 'Nuff said.
Finally, in the confusion of CQB, it's not unknown for mistakes to be made in the heat of the moment. I've personally witnessed one guy going into a house-clearing firefight carrying an R4 (South African-manufactured copy of the Israeli Galil) in 5.56mm. He came out a couple of minutes later carrying an AK-47, in 7.62x39mm. Needless to say, he didn't have the right ammo or magazines on him... We gave him all sorts of hell for his mistake, but we all learned from it. Use of slings increased dramatically from then onward!