Think it through Braun.
If you ditch ANY weight, you now have imposed an artificial GAS FLOOR on your dive.
Use more than that much gas, and you now have an uncontrollable ascent as you come up. Exactly where it is you cannot compute, but that it will happen is a certainty.
An uncontrollable ascent is one of the most dangerous things you can experience as a diver. It can and will either bend or embolize you, and while a bends hit is "fixable", an AGE hit might not be.
Undertaking an act that will cause such an imbalance in your kit is, IMHO, unwise in virtually all circumstances. I agree that if the alternative is CERTAIN DEATH, you might choose this (e.g. you're in a "no floor" environment, sinking fast, and MUST ditch mass RIGHT NOW to avoid going to the bottom of a 2500 foot trench!)
I would, however, also argue that if you find yourself in that situation you have not correctly planned your dive. In such a dive situation you need redundant buoyancy of SOME kind - whether that "some kind" is a redundant BC, a SMB, a drysuit, or something - you simply must have SOME kind of redundant buoyancy if you are diving a "bottomless pit."
Let's take the more common scenario.
You are diving Nitrox to 100-110', with doubles as this is a planned deco dive, with a cylinder of either 50/50 for deco or perhaps a cylinder of 100% O2 for the final deco at 20', planning to use backgas for your deeper stops. The dive is a wreck, and you plan some penetration. You are in a relatively thin (3 mil) wetsuit and weighted for neutral with empty tanks at the surface, as you should be. The water temp at the bottom is 75F, which is fine, and 80ish above 40' - normal for this part of the country.
With the tanks full, at the surface, you are -18. Why? That's how much the gas weighs. Cool.
(Note that with trimix you shouldn't HAVE a mass-of-gas issue, so this situation should ONLY arise with Nitrox - thus, the depth range I'm referring to here. With 30/30 Trimix on this dive you'd only be -7 or -8 - not a problem.)
Ok.
You get down there to the planned depth, within your MOD, and your wing comes apart. You now have no buoyancy, your gas, nearly all of which is in the tanks still makes you -18, and worse, your suit, which is usually +10, is now +3. Your total negative buoyancy is -25.
You can swim up (which you know, because you've done it to test) a kit of -15, at least for a short distance. You need to ditch 10lbs to be able to get out of this pickle, or have a redundant source of 10lbs of buoyancy.
You have the following options:
Your buddy should have a working BC. If you "link up", you can both use HIS BC to rise from the bottom. This is not a zero-risk game, however, as he is now effectively using his BC as a lift bag - to lift YOU! That's dangerous for him if somehow you become "detached", as HE will rocket to the surface, and YOU will sink like a stone. This option sucks, although its a useful "last ditch" option.
If you have a 50lb bag, you can hook it under your left (useless, remember, as your inflator doesn't do you any good) arm, put in some gas, get back the 10lbs you need, and start to swim up. You do not attempt to use the bag as a BC replacement, because you don't have that fine of control over it! Rather, you use it ONLY to offset enough negative buoyancy to be able to SWIM up, and dump it early and often. When you perceive that you're a few lbs negative (instead of -15!) you dump some of the bag's gas, to avoid IT running away on you. That's option #2.
You probably do not HAVE 10lbs of ditchable weight. That one's out the window.
Your other option, and the one that makes plenty of sense, is to dump 10lbs of GAS. You will still have 8lbs of GAS, which is more than 1/3rd of what you started with - remember, you planned with a "rule of thirds", and you're well within that. You have no deco obligation (remember, this problem only "bites" you if it happens at the start of the dive - when the tanks are full!), and thus, having dumped the gas you have more than enough for a safe, controlled swim-up ascent, a safety stop, and, lo and behold, when you get back to the surface your kit is only -3 or so - easily finned to stay on the surface. If you have a few lbs of lead on your belt you can ditch THAT at the surface to insure you don't sink, and all is well.
Now let's say you were diving a really thick wetsuit and you had enough lead to ditch that instead.
Ok, you ditch the 10lbs of lead. Now you start to kick up. As you do, your suit uncompresses. Also, you now are consuming gas. You MAY find yourself above the "critical depth" - a depth you cannot possibly calculcate accurately "on the fly" - and if you do, you had better have something to hang on to (like an anchor line) or you are going to the surface RIGHT NOW.
That's the risk of this approach, and why I'd only use it as a last, final resort.
