The reasoning behind it is that they feel that it is too much task loading for some divers. Especially new ones to control two buoyancy sources. And I have to say I agree. If you are not taught proper buoyancy control in Ow class and tested on task loading exercises then it may be. Of course if that is the case you have no business in OW anyway. Using two sources of possible control is NOT too much task loading for the diver of average intelligence who has been properly taught in the first place. Problem is many students never hear of real buoyancy control and trim until much later. They get screwed in this area. Hence the fighting for control, swimming vertically, and not knowing how to properly weight themselves in the first place. All things that should be covered in OW class. When discussing drysuits in OW class I tell them how most divers use the bc for buoyancy and suit for squeeze or a combination at depth. As said before if properly weighted the amt of gas for buoyancy is about what is needed to control squeeze. It is truly insulting to student's intelligence when they are taught by the book and nothing else and they find out the hard way. But that's what you get when you dumb down courses and shorten training times.
