So I'd suggest that there is no reason for a 'rule'. Everyone doing this sort of diving should be able to work it out for themselves. If you can't, then I do believe the label 'Idiot' may be appropriate...
As an aside, if I have a 6 lbs weight belt, or 6 lbs of air to dump, Im in favour of ditching the weightbelt... I like my air
First, if you are only six lbs negative, you can swim that up. If you can't, you shouldn't be in the water in the first place.
Let's assume instead you have 18lbs of gas - not implausible if you have on a set of doubles with Nitrox on them, and a set of HP120s doubled (which are -4 or so empty.)
You also have 10lbs of lead on you, to compensate for a thick, 2-piece 7mil wetsuit (along with your BP, which is -6.)
Now, you get to the bottom and the seam on your wing lets go, making it useless. You now have no buoyancy, and are something like -20 due to suit compression and the 18lbs of gas on your back.
There is no way in hell you can swim that up.
This is the "nightmare scenario" that people are talking about.
Now, you can drop the 10lbs of lead. But if you do, when you rise up you had better hope you haven't breathed more than 8lbs of that gas, because if you have, you're not stopping on the way up. That's not acceptable.
So, you get down there, have this problem, why not vent 10lbs of GAS? You still have more than 1/3rd of your gas. You can now swim up the rig, and more importantly, as you get near the surface your rig remains BALANCED, which means you can make a safety stop, control your ascent, and ONCE YOU GET TO THE SURFACE ditch your weight belt.
What good does the extra gas do you if you embolize due to an uncontrollable ascent?
Think outside the box folks. The 'one true way" mantra is BROKEN in these situations. You must ditch SOMETHING to swim the rig up, but why would you ditch something that will make you inescapably buoyant once you begin that ascent? That's STUPID! Instead, ditch something that you intended to get rid of during the dive ANYWAY, and that will NOT impact the balance of your rig.
You only have one thing that fits that description, and in this situation you have way more of it than you need with you.