I believe it is essential teach CESA, and I believe the vertical performance on the OW dives is important for that. However, I have a lot of trouble with the way we do the horizontal version of it in the pool.
For those of you who suggest "bring your question to PADI," I did that. About two years ago I had an extensive exchange about this. They did not change my mind, and I did not change theirs.
I was a part-time athletic coach in a number of sports for about a quarter century, and one of the most important things I learned during that time is to make your practice "gamelike." Many athletic coaches actually harm player performance by having them practice skills differently from the way they are performed in the game, thus ingraining poor habits. (If any of you coaches out there want examples, PM me.) We do the same thing with some of what we do when we teach CESA.
First of all, by having them do it horizontally in the pool, we teach them a different need for a rate of exhale than they will have ascending vertically from depth. To cover 30 feet while exhaling, students have to limit their exhale severely. This teaches them this is a really hard thing to do, when in reality it is much easier in to do vertically because of the expanding air. While you might think that is good, we have planted a "this is really hard" notion in their mind that might re-emerge when it is time to use it for real, leading them to do what you don't want them to do--hold their breath.
Now let me combine two other non-gamelike things we have to teach, both of which have been mentioned in this thread.
- First, we make sure they have full lungs before they start.
- Next, we fail them if they have to take a breath before they are done.
The full breath one should be obvious, but many people do not realize one of the reasons why it should not be an issue. If you have exhaled all your air, there is still enough oxygen in your blood to get you safely to the surface without inhaling. You will have a monstrous urge to breathe when you get there, but if you understand that and don't do something stupid, you will get there.
You have to give it some thought to realize why the second one is non-gamelike. What would happen in the real world if the ascending diver did not have a full breath and had such an urge to breathe? He or she would try to inhale, and what would happen then? Well, assuming the diver had properly retained the regulator, he or she would get some air! The tank was not empty--it just did not have enough air to deliver it at ambient pressure. Once the diver has ascended, air will be available.
Thus, inhaling at that point would be the proper thing to do, but we fail the student who does it. Once more, we teach a diver who is someday in that position that he or she cannot make it if they don't have enough air, making it more likely that the breath will be held.