It can come out FAR more easy than it would if it was in a backplate pad or a pocket.
I can only talk on the basis of my own experience - but I simply don't agree with that, in practice.
DSMBs can drop down from the 'H' Pak pretty easily - it happened occasionally for me, especially as the pak got more and more 'well used' (the elasticity going at the sides). Having done a fair few dives with both methods, I just don't notice that either method has a significant 'trend' towards dropping the DSMB.
With bungees, you can
see how well secured the DSMB is when kitting up. Mine is rolled up, not attached to reel... or to me. If it drops out, it ain't gonna cause much problem except for the loss of it. Bungees make it far easier to re-stow the DSMB if it does slip though. I can 'feel' the DSMB at the base of my backplate. If it does slip, then it is only from one loop. I can feel move... reach back, one-hand, and re-insert it into the bungee loop. That's a definite advantage over an 'H' Pak.
As critical as an SMB is in the ocean, and as critical as not having things come loose in a wreck would be, I would think having it in a secure location till needed would be smart. I recall a dive where a buddy of mine had a bungeed SMB come unbungeed. He had a tail for a little till he realized (shallow OW dive, I just enjoyed the lulz).
Again, that's a presumption that two loops of bungee is 'less secure' than a pak. When mine's dropped out, it never became a 'tail'.. it's rolled/folded up neatly and that's how it comes out. It doesn't unravel, or stay attached to me.
I can't claim to speak on behalf of other divers, who may causes chaos with DSMB 'tails'... but I've found that some precision and attention to detail with kit stowage eliminates this from happening. As this is the DIR forum, we are - of course - assuming that the divers concerned are competent, properly trained and observant of the small details?
The only times it's ever come out for me, has been on training dives, where I've been doing kit remove/replace underwater etc. Honestly, they drop out of the Pak just as easily under those circumstances.
Spools routinely come unclipped and/or dump line everywhere (which is the reason we keep them in pockets),
Define "routinely"?
For me, "routinely" would be about once every 2000 dives. Is it more often for you? Why?
Taking a second to stop, stabilize, assess the situation, and retrieve a spool would be time well spent, imo.
...and drift further down the corridor, past every junction that now exponentially decreases your chances of finding an exit/regaining the main line etc?
...whilst your lost buddy slowly moves away from the line in zero viz, increasing your search area with every second that passes?
I don't believe in rushed procedures... but add current to zero viz...along with multiple routes/exits/complex interior layouts... and time does indeed become critical.
The technique in that situation wouldn't be to flop everything out of the pocket, but to simply reach in and unclip the needed spool and carry on. I can manage to retrieve and replace spools from my pocket in very short order, so I don't think a time argument really holds true (esp if you plan your gas right).
But I thought you had 'everything' loaded in your pockets. Perhaps I'm lacking finger dexterity or something? I pack my pockets in a consistent order - left side is 'working' and stuff is stowed in the order it gets used - line markers, jump spool, wetnotes etc. Right side is 'contingency' and is stowed in order of emergency precedence - safety spool, back-up/back-up torch, back-up slate/notes etc.
With loaded pockets, I've never found it easy to 'reach in' and select a single item for removal. Especially not when in zero viz and under pressure. Or when wearing gloves. Or when I'm in a confined space.
As mentioned, time argument isn't really about gas supply... although all those little delays do add up.
...this is all based upon the presumption we are talking about worst-case emergency operation....