There are many ways of steadying a rifle, so why consider the sling? It’s not a be-all-end-all solution. It’s just a tool, so let’s go over some of the other tools available, and explore some of the advantages and disadvantages of each.
There’s the bench, which doesn’t go all that well into the field, so I’ll just dismiss it outright as a practical implement for steadying the rifle. I suggest you do too, unless you plan on hauling the shooting bench with you when you take your rifle to the field.
The bipod is a great tool. I won’t disparage bipod use, because it works great, and people are getting better at using it. The primary issue that will preclude the use of the bipod is intervening terrain, e.g. tall grass, small hills, etcetera, that obstruct line of sight. Bipods can be used in positions other than prone, but there’s the issue of the necessity to find a suitable surface to plant it on. Like anything else, training with and using a bipod will increase your ability to use it well.
Another useful method to steady the rifle is to take advantage of what I call “improvised support”. This involves finding stuff to rest your rifle on. It helps, and I recommend taking advantage of whatever you can use to get steadier. You can’t count on being able to find it when, where and how you need it, and finding it can be a bit of an art. That is the principle weakness of improvised support.
There are several advantages that a sling enjoys over other methods of steadying the rifle. It’s lightweight and easily portable (a bit more so than a bipod). Nylon, especially, adds very little weight and is very durable. It serves double duty as a means for carrying the rifle. It tends to stay on the rifle, so it’s quick to get into action, provided the sling design allows for quick use (the bipod is similar in this respect). The sling can be used in any position in which the support arm can rest on something, such as the ground or part of the body, e.g., the support side leg. The sling can be used from prone up to kneeling, and can be combined with the use of improvised support. Some use the sling in standing as well. The method of using the sling is consistent from position to position, which makes it easy to use when other methods may present greater challenge to adaptation.
Take the following photos as examples. Let's say the shooter sees a target of opportunity in the areas indicated by the red circles:
Getting prone won't work because of the intervening terrain (hills and tall grass respectively). There is nothing to use as improvised support in the first photo, and nothing nearby in the second, especially considering that the close range and open terrain will likely require speed before the target decides to leave. If you have one extra second to get steady, the sling will be of use (not all slings are that quick, including the one pictured above).
Disadvantages of the sling? It doesn’t stabilize quite as well as a bipod and doesn’t allow you to see your impacts (not the same as calling your shot). Although it’s very simple, it’s still subject a man-made object that is subject to failure, so it shouldn’t be relied upon as a crutch any more than benches, bipods, or even firearms. The sling can snag when carried in dense foliage. The best advise if this is a problem is simply to take it off and stow it temporarily in a pocket or pack. You’ll probably be snapshooting at a close range target anyway.
What about relying purely on good positioning with no extra tools used to steady the rifle? I think it sounds like a good idea to practice it. I have to admit though, that I’m always looking to use whatever is handy to gain an advantage. But sometimes all you got is what you got, and it’s good to be able to get by without extras. In my experience, however, the sling will tighten groups up considerably. It’s hard to put a number on it, but I would say that the sling could potentially double your accuracy over using nothing but good position.
Kneeling without sling, 100 yards, 6/20/11:
Kneeling with loop sling, 100 yards, 6/20/11: