Getting hurt is an integral part of every roleplaying ruleset that even approaches simulation. We need ways to figure out how to be hurt, and what exactly happens when our characters are. All simulated fights require a method of dishing out "damage" of some form or another. For AD&D (and all the D&Ds, really) that method is the slow whittling down of hit points until you drop dead at zero. Older gamers (those who tout the OD&D style of play) tend to prefer the very simple rule of zero being death. However, I grew up on Second Edition and there was a rule that we used all the time, mostly because my games would be too lethal without it.
One out of four characters dies before reaching level two in the games I run. I've heard this technique referred to as the "character grinder" or the "character funnel," as those who survive that first winnowing generally make it to higher levels. The rule that allowed me to keep that ratio instead of, say, three out of four, was the Dead at -10 rule. I realize that it seems to run counter to the kind of playstyle I claim to espouse: gritty, dangerous, deadly.
However, here are some reasons why I still use Dead at -10, beyond lightening my kill ratio:
The most common form of battlefield death is that which results from bleeding out. The human body is actually incredibly resilient. In my time studying late antique skeletons I have seen men who have suffered multiple sword-cuts to the head and gone on to live (who knows how happy their lives were, but that's immaterial). The wounds had healed, indicating a long life after the cut. If someone can be tended to relatively quickly, death can be avoided in many cases. This is a strong bit of simulationist support for the -10 rule.
Now, I've altered it to make it a little less forgiving than it is: characters in my games bleed not at a rate of 1 hp/round but at a rate of 1d4hp/round and they must still roll their initiative to see when the blood loss occurs, meaning a sequence of shitty rolls can very easily spell their death. The rate of blood loss is even higher if it's a critical hit which put them under (which it often is) because of the bleeding rules associated with the Combat and Tactics critical hit charts.
Furthermore, the rules on getting back up after being dropped to negative numbers are, as written, much harsher than I've ever seen anyone else use them. While it is a boon to be allowed to bleed into the negatives, it is much more gutwrending to try to save someone who is bleeding to death than simply walking over to them and staunching the bloodloss.
Yes, you can simply bind the wounds, but that will leave the unconscious ally at whatever negative number he's at, Hovering on Death's Door (as the rule itself is actually called). He will be unable to awaken, move, fight, or do anything on his own and will need to be carried to safety. Any wound suffered will set him bleeding again and likely kill him outright (after all, how much of that tiny negative hp buffer could he still have left?)
Attention from a physician (anyone with the healing proficiency who makes their check) or any type of healing spell (short of a heal) will, over ten minutes, bring the wounded person back around at 1 hp, unable to fight or use magic, stuck at 1/2 move rate, and very much crippled until they have some real rest. Further healing of any kind short of a heal spell will do nothing. Potions of Healing and Extra-healing are worthless, and physicians cannot heal any further wounds until the ally has rested.
Yes, this is a character that would otherwise definitely have died. This schema transforms several certain deaths into not-so-certain deaths. I know that this interferes with the lethality of the system, but it also introduces an entirely new level of drama and danger in fights. When your friend drops, he might not be dead (he might still be dead, though) and rather bleeding to death on the ground. Most intelligent foes will leave someone who falls in order to concentrate on those who are still alive and dangerous.
Of course, this can start to happen very often if players are always attempting tasks that are beyond their reach. I am not outside of doing unpleasant things to those who are always falling down: increasing the bleeding die for grotesquely large wounds, simply declaring people dead if the blow seemed particularly gruesome, reducing ability scores permanently on revival, passing out scars and debilitations.
I prefer to let my PCs Hover on Death's Door... it makes it all the more important when they cross it. And, as I've mentioned before, most deaths in medieval combat would have occurred not from instantly fatal wounds (head cut off, stabbed in throat, etc.) but rather from slow bleeding that, had there been magic or battlefield medics, may have been preventable.