As faethor mentioned, they can be a bit of a pain when you transition down from the carbine, it will whack you in the jewels if you have to run and do anything else but hold onto the rifle, also you do stick the muzzle in the ground a lot when you do go to the ground with someone... Just recently in the training house I saw a guy nearly KO himself during a takedown when the muzzle stuck in the pads next to his knee and drove the butt of his rifle into his jaw -- ouch!
Do not make one that hangs the rifle at the center of balance, seen that also and well, imagine running through a house after someone with a rifle hung horizontal at your waist.
I use an old school "ring thing sling" for a single point, mostly out of habit I guess, but I let mine fall to the weak side and it seems to be much better, slower to get back to, but it is just how I roll I guess -- I never have understood the guys that hook the rifle behind their pistol, may look cool on the range, but now your reaction time with both weapons is slower?