@KWS it's all about logistics when doing extended penetration as
@JohnnyC and I tried to outline. In OC long penetration dives, you have to breathe the gas both in and out. Pick some BS numbers but say an AL80 is good for 1000ft on OC with you breathing a third in/thirdout/third reserve, you determine it's good for 1500ft on CCR based on the same parameters, and have a 10,000ft penetration. Your primary backgas/bailout is say LP85's which are comparable to 3x AL80's.
That dive says on OC, you need 7x al80's+LP85's, on CCR you need 4x 80's + LP85's. Essentially save 1 bottle since a CCR is comparable to a set of doubles. Looking at bmccr only for this.
For the OC dive, you could do a setup dive in a couple ways, but the way I would do it is to drop a full AL80 every 1500ft and drop 4 of them as safety/bailout bottles, same as you would on CCR. For the CCR diver when your doing the actual dive, you have the CCR and a pair of LP85's and you're done. Everything else stays in the cave and happy days. For OC, you need the LP85's, and still need to carry 3x AL80's for penetration to drop off when they hit 1/2+200 ish. Bit more complicated with bottle drops, gas switches, etc.
If something goes wrong on the CCR, you still may not have to touch those al80's though because you have SCR mode which can get you a 10x or more gas extension. You stay on the LP85's the whole way out and happy days.
If something goes right, you can still poke around on any leads or jumps on your way back. You have safeties out to 6000ft, and as long as you don't exceed your 10000ft total distance from home, and/or 4500ft from the gold line, you can go wherever you want in that cave.
On your example of why it was explained that they're unsafe. It sounds like it was explained by someone who never did actual gas planning and tried to use similar rules of thumb like we use with OC. So many cave divers did 1/2+200 and didn't touch their backgas which works with 2 stages and 104's on a Gavin or UV26 going 200fpm, many others did 1/5th's which works with the same scooters. That "rule" doesn't work on an XK1 or a Magnus going 250fpm. Those same divers tend to not have any idea what their actual sac rate is, or what their actual kick speed is and started using all sorts of "rule of thumb" kind of tricks for gas planning instead of actually doing the math.
DPV's and CCR's allow you to exceed your bailout gas, but to
@victorzamora 's point, only if you're an idiot that doesn't actually do the planning ahead of time. Do it based on distance markers or known points on the map if you have them, also know how to do it based on time if those aren't available *have to know how fast your scooter goes and how fast you go though*.
Frankly I think DPV CCR cave dives are a lot safer when planned properly than kicking OC dives to the same penetration distance and you certainly get to a point where the distance/depth makes OC more dangerous from a quantity of gear perspective
