DiverBuoy once bubbled...
One can descend vertically just fine .... but one must be able to arrest one's descent BEFORE touchdown and negotiate into a horizontal postion. I teach this technique.
The vertical body position during descent allows eye contact mask to mask with your buddy in low visibility and allows the diver to ascend a few feet very quickly if they experience a squeeze. It allows the diver to clear a mask easily if it begins to flood more rapidly with each foot of descent. Buddys are in an immediate hookup position for emergency purposes (such as alternate air source use).
All this talk about horizontal DURING ascents and descents scares the crap out of me, but as long as you haven't lost anyone (yet)...
We have eye to eye contact when horizontal. In fact in this position we can move closer or further away as needed with a single flip of a fin. We share air just fine in the horizontal position (better for swimming in) We halt or reverse our ascent as needed with buoyancy control. In the horizontal position we land where we choose not where we fall.
Why would we lose someone descending horizontally?