This stems from an instructional K.I.S.S. philosophy. No ifs, buts or maybes. Don't hold your breath. That's simple, intuitive and easy to grasp and easier to remember.
Whilst more experienced divers might debate the finer points of "glottis positioning during paused respiration", the novice diver needs not be confused.
The novice diver is, after all, the one most likely to suffer stress and confusion in the water. Developing an ingrained response to never hold your breath might save your life one day. When panicked, there can be a tendency to hold the breath. Initial guidance and instruction aims to over-ride that tendency. For that reason alone... keep it simple... develop a conditioning to never hold your breath.
IMHO, it is irresponsible to input any positive encouragement, discussion or speculation about 'open-glottis respiration pauses' into the 'Basic Scuba' forum. Such considerations are the preserve of experienced divers - those who've done a few hundred dives, dealt with a few problems...and developed sufficient instinct to eliminate any likelihood of screwing up an ascent during high stress loading.
---------- Post added May 9th, 2013 at 04:51 PM ----------
If I exhaled hard to descend, like if I were carrying the bare minimum amount of weight, and then I held the exhalation all the way down, could I theoretically collapse a lung as that small amount of air gets smaller under pressure?
Given the extreme depths that free-divers (breath-hold) attain, I doubt that'd be a problem. That said, some pretty weird physiological occurrences happen on those deep breath-hold dives. I'm not sure how initial descent with compressed air might effect that.
Luckily, there's no good reason why a SCUBA diver would ever need to do that... so from a practical perspective, it's irrelevant.