I completely agree that many new divers are over-weighted and carry more ballast than they need. This complicates ascents and makes buoyancy control more tenuous. This is primarily because the excess weight causes them to “manage” a bubble in the BC that is larger than would be otherwise needed.
However, I know quite a few very experienced divers who clearly are diving with a modest amount of lead that is greater than would be needed to hang neutral at a shallow depth with their tank low. Why would a diver choose to wear more lead than the absolute minimum?
I can think of a few reasons, one primary one is the ability to drop to the bottom and be stable in a high current environment. Being 6 or 10 lbs heavy on the bottom can greatly reduce exertion by the diver.
Another reason, might be to be able to hang heavily under an smb at the end of the dive and still be able to completely relax and pull the smb down so that one end is submerged and the other end is near vertical. That can be a strenuous task if the diver is neutral or light at 10 or 15 feet.
Another potential benefit of carrying a little extra lead is that it allows the diver to descend more rapidly and with little or no kicking, which can reduce workload and probably air consumption. Of course, this practice presents its’ own challenges, dangers and is presented as something to avoid when new divers are being taught. That doesn’t mean that I don’t see some experienced divers doing it however.
Also some instructors and guides might carry some extra lead that they can donate to a diver who finds himself light. It is often much less inconvenient to pass off a 3-lb weight at the surface (or just below) than to abort the dive, reboard the boat, add lead and then start a new descent.
Wearing extra lead, means the diver must manage the ascent more carefully and the extra lead will be “paid for” by a little extra drag associated with a slightly more inflated BC, so a diver probably should have a good reason why they are deciding to wear more lead than they absolutely could get away with.