Steve - I think your right if you just drop a down eyed hook with a bead on it in a tank of still water with no line attached, then yes I think the hook would drop, hook up to the bottom.
But for sub-surface flies; add a line with tension and angle, moving water (up and down stream), fly design etc, etc..... It gets complicated in a hurry. I think the big thing is the balance of the fly. And what effects that balance, that determines how the hook rides. Whether it's; hook shape, beads, fly design, line tension, water current, etc..... it's all about the balance of the fly... It's easier to just let the fish be the judge or tie everything "in the round".