Although the training tends to get a little dumbed down in practice, the PADI drysuit course actually says to use the suit for buoyancy control with shell suits, and to use the BCD with neoprene suits or any tec diving, adding just enough to the suit for comfort. Maybe everyone learns in one type, and then remembers that as the only way they were trained to do it.
I have learned to do both with my Fusion suit, as I dive a bit heavy to have some extra weight I can clip onto a student in a pinch. If I just use the suit when overweighted, I tend to burp the extra air bubble out my neck seal, and let some water in.