Hmmm. I personally would find it a bit scary to consider part my life support system as "ditchable" weight. Is this really taught or endorsed by anyone
In a case of emergency, dump all air to float up
OK, enough joking. The question is valid. Mandatory stops also complicate things. Let's get serious:
Endorsed by anyone? Probably not. It would be a desperate deed best avoided. I dive mostly in mines, and also under ice, so inadvertent dropping of weights is really not an option, and I have tried to make sure that the weights stay with me no matter what happens. There are other more controlled ways to reach the surface.
There are a few failure modes that are relevant to open circuit scuba:
1) loss of existing buoyancy: torn bcd bladder, flooded drysuit, ...
2) loss of bcd inflation during descent
3) drifting at sea
A second
big enough independent bcd protects in case 1. It is quite unlikely that one would loose both a drysuit and a wing (or two independent wings) simultaneously. I mean that they would start leaking at the same time...
The second case is a nasty one. A few scenarios that I can think of are a) running out of air in
both cylinders during descent. Also, b)
two burst LP hoses, c) or
two burst tank o-rings, d)
two first stages freezing shut at the same time, or some other
dual failure could lead to uncontrolled descent. None of these situations seem very probable. Descending extremely fast could lead to problems (of not beeing able to inflate a wing fast enough - there was another thread about this). We avoid this by descending slowly, right?
One should be facing and communicating with a team member throughout the descent, in any case. Descent is the most dangerous part of the dive. Unfortunately, many commence "team solo descents", and often overweighted. It is a very dangerous practice that has killed people.
I don't dive at sea so #3 is not relevant in my case. Ok, I admit: I have been to the sea twice in sidemount gear. I did carry 4 pounds of droppable weight in the front on my waist belt. It is not a lot, but it's something. I would have needed to open the belt buckle though, to drop that.
losing all your trim buoyancy early in a dive at max depth, then ditching enough to be able to swim up to your deco stop(s) and hold them.
This sounds like a single bcd bladder scenario. No-one in their right minds would dive without droppable weights (
and the mental state required to actually drop a weight - do practice it) using a single bcd bladder only. We have all seen the video of the last dive of
Yuri Lipski, haven't we?