Kraken, I agree that good buoyancy needs to be emphasized from the beginning, which we do. We also teach a proper weight check, and we try to get them done with all the students during OW. However . . . the weight check on our OW dives is often not really very valid, because the students are anxious enough to hold a lot of air in their lungs, and off balance enough that they don't float the surface without kicking some. If we are fortunate enough to have a student who makes it through a formal weight check properly, it's often the case that they can't descend with that weighting, due to anxiety and improper technique. Thus, even with our classes, which do not involve kneeling underwater at all, students can end the class several pounds overweighted. And although we try to move their weights around if there are gross and obvious problems, there is simply not time to fine-tune static balance during a class where the students often can't stay still for very long at all.
Once the person has a few dives under their belt and has been able to relax a little, and particularly if, in the meantime, they have acquired their own equipment, these checks and adjustments can be refined considerably. In addition, buoyancy practice done with an extremely stable and solid visual reference is priceless, or at least I found it so. There are a lot of tips for good buoyancy and trim that we offer during OW, but the student often doesn't integrate, just because of the amount of "stuff" they are trying to remember. Going over that again in a PPB day is an opportunity to reinforce it, and have more time to show the effects of small changes, since the student doesn't have to spend underwater time doing air-sharing and mask clearing drills.
So yes, buoyancy should be taught in OW. But since, ten years after mine, I don't think my buoyancy is perfect, I don't think it's unreasonable to say that virtually all divers benefit from time and effort spent to improve their buoyancy and trim, and especially the typical fairly novice diver who takes a PPB class.