Very little mention is made during basic SCUBA training of trim. There is a reason for this - training agencies tend to assume that divers will be wearing a stab jacket, which is not something that lends itself too well to trimming. And it's only in recent years that the weight belt has been supplanted by the ever-more-popular harnesses available now. The agencies have simply not caught up yet.
There is also the fact that many people choose to control their buoyancy using their drysuits. This means that their bouyancy is spread out fairly evenly over their body, and the weight-belt in the middle does actually lend itself to reasonably good balance.
When I forsook my Buddy Commando for a wing with a backplate and harness, I was already aware, from my reading about the subject, that 'trim' was considered very important by other wing-users. When I started trying my wing out, I could understand why. It took me hours of in-water playing around to get my weight well-distributed.
The sad truth is, a wing is much harder to get set up than a stab jacket. Give someone a weightbelt and a stab jacket, and their trim will never be terribly good, but it won't be too bad either. This is not true with a wing - whilst your trim with a wing can be better than any stab jacket will ever be, it takes a lot more time and effort. This is something that many people who try wings out don't understand. They expect it to be just like a stab jacket, find themselves unable to stay horizontal, and blame the wing as being no good.
As an example of how different wing and stab jacket trim is: When I dived with a Commando, I wore a weight belt with 24 pounds of lead on it. It was fairly comfortable, I never had any complaints.
My weight is currently spread out as:
So I am actually carrying slightly less lead, and my twin 7s are a lighter setup than my 15 + pony ever was.
The benefit of the whole setup is that I barely notice that it's there when I'm in the water. I float perfectly horizontal with no finning necessary, and the weight is spread evenly across my entire upper body, instead of hanging off my waist.
There's also the fact that, using the wing for buoyancy, I don't actually have as much strain acting on me as other setups can put. Assume you have someone who wears 20 lbs of lead, and carries cylinders that contain 8 pounds of air.
At the start of a dive, the scenarios are:
That's where having your weight integrated with your BCD scores: Instead of having your buoyancy pulling at you while the lead pushes, the BCD pulls at the weight, cancelling out a lot of the weight, and taking a lot of strain off you. In the above example, the integrated strain was less than half of the weight belt strain - another good reason to get rid of weightbelts.
Weight-integrated stab jackets, while not perfect, are very comfortable underwater. A wing and backplate, however, is even better. You can put as much weight as you like on it, absolutely anywhere you like. No integrated stab jacket can make that same claim. Thus while detractors of the wing claim that you sacrifice a lot of comfort and stabilty, this is founded on a misconception. The versatilty of the wing that allows it to be made superbly comfortable underwater, sadly also allows for it to be terribly uncomfortable underwater as well. If you intend to switch from a stab to a wing, bear in mind that you have a lot of work to do before the wing will really work for you. It's not a "Try it on and see if you like it" piece of kit.