Personally, I don't have a problem with slings. They're commonly found on guns... I think they're within the rules.
However, I'm not sure they help with shooting, except perhaps in the kneeling position. I think most tend to rely on them in the standing position and i'm not sure it actually does any favours... in fact i find my standing is better free than even if i'm leaning on something !
I think most people i've seen use a sling have dropped them after a time... but then we all need a crook now and then