Ian
True enough theres been numerous stories of divers getting into trouble using the small independent bcd bottles, just as there has been with other equipment. Clearly to the unwary you can end up in a stack of trouble.
The background to these bottles, evolved from the days when direct feed was not around. In the early days Bouyancy control was achieved by blowing air into the BCD via the mouthpiece and dumping it. If you didn't fancy the idea of getting a mouthfull of sea water the option was to fill up your bdc with air from your bottle. As thankfully direct feeds became common place these skills became redundant.
Already been mentioned, some of the skills assessment for qualification included cracking open the bottle & venting air into your BCD and breathing it from the bag, ain't that grim!!!!!
My personal experience with it,
Years ago having lost contact with my buddy I got myself into a situation at depth with little bouyancy and unable to lift of the bottom.
I was 100% sure I wasn't going to reach the surface. Dumping the weightbelt and inflating the bcd with the inflator didn't have the effect. My A*se was tweaking & I desperatly wanted to see the light of day again. I cracked the bottle much to my surprise nothing "seemed" to be happening, then 5-4-3-2-1- lift off, probably something akin to those guys that do the submarine escape drill in the tower thing transpired. I vividly recall the surface coming into view at an very alarming rate and managed to slow the ascent with a combination of desparate dumping of air and flaring.
I guess I was fortunate as I only had a few mins bottom time run up and I other than a desire for a clean change of underwear,having "nearly Shit myself"
suffered no adverse effects.
Probably on the basis of my unfortunate experience, you'd be forgiven for thinking i'd still be using one NO, got rid of it yonks ago. I remember when I first removed it the act nearly caused a riot in the local dive club guess it was "then" considered an unsafe diving practice etc etc.
Footnote to this is an extract from a well known diving book,
recounting a venture into the whirlpool of Corryvreckan.
A proffessional Scallop diver dropped for scallops at 20m, he found no bottom at 40m so finned up, but found himslf still going down. He therefore fired his ABLJ ( Suicide bottle) and found himself still going down. He saw the bottom at 75m! it was going past very quickly, but then at last his bcd finally pulled him towards the surface.
Tony