This wouldn't be the first game rule to occur to me in a dream ... but then, I was on a percocet cloud when I conceived of the mass hit points per hit die rule, those being the pain killers I was on when I snapped my quadraceps tendon back in '08. Take note that the linked post was written on August 12; on the evening of August 8th I snapped the tendon, on August 9th I was in surgery and the night after the surgery I thought the idea up ... then had to wait three days to have the strength to write a post about it. Wrote that post laying on my back, touch-typing, percocet knocking the pain down.
About five hours ago I woke from a dream where I was sitting at table, running a game (yes, I do this in my sleep, too), spontaneously coming up with this combat suggestion. I don't think I'll run it though; my combat system is difficult enough as it is, and it is nothing like as good an idea as the mass thing was. Still ...
There's Boromir with three arrows stuck in him; Inigo Montoya with a dagger in his shoulder; Carrie White with a dagger in her back; King Arthur in Excalibur working his way down the spear to put it into Mordred. Images of people with weapons stuck in their bodies. Where's the rule for that?
Now, only a couple of days ago I was mentioning that hit points were exhaustion - except, of course, the last ones that actually kill the character. And I've just said I'm not going to use this rule, so I'm not screwing with my own perception ... but still, suppose we consider a rule that allows for certain weapons to remain in the body - stabbing, bladed weapons, including the spear and javelin, but discounting anything fat and blocky or heavy and cumbersome. The obvious mechanic for it would be the natural 20 on the die, indicating that the instrument thrown or used has sunk into some part of the body that - if it doesn't kill - pierces and sticks. Through Inigo's shoulder, say.
For anything thrown, that's simple enough ... but suppose the option for a stabbing weapon such as a sword or spear is to leave it in.
Most of my parties carry numerous weapons, so it wouldn't be any problem to pull something else; it might even increase the benefit of those weapons, so that the spear in particular increases in value not because it does more damage, but because it's harder to pull out than a dagger or a sword. I've always felt the spear got short-changed in the game.
Of course, it does say the uruchai rolled three 20s in succession when he shot Boromir, but I've had party members do that in a fight. Not impossible.
Rules, then. Does the weapon cause damage while it remains in the body? I'd say yes, if the victim continues to fight or take action beyond slowly moving away. A small weapon (dagger) would cause 1 damage per round; a medium weapon (short/long sword, javelin) would cause 2; and a spear or pike would cause 3 (mostly due to the long handle of the weapon which produces greater stress on the internal musculature/organs).
What effects does it cause to fight with it? I'd rather not increase the effect of the larger weapons by giving them a higher negative modifier, also; perhaps a random die roll, a d4, could be rolled for any weapon to determine the particular encumbrance the weapon is causing, thus removing the necessity of hit location and the like. "Ah, the spear seems to have gone through nothing vital, you're -1 to hit" or "The dagger has clearly hit something important; you're -4 to hit."
Difficulty of pulling out the weapon. A dagger takes half a round (in my game system, 2 action points, but that's only going to mean anything to my players); the medium weapon, a full round to remove it. For the spear, 2 rounds, and for the pike, perhaps 3. But this could be worked out by a random roll also. Keep in mind my rounds are 12 seconds, so if you're still using 1 minute rounds, it would probably be equal across the board.
Damage upon removing. It's tempting to say it causes a wound, but I have a system for that already, which I don't want to increase. I might argue for my own purposes that the wound doesn't start bleeding (-1 hit point per round) until the weapon is actually removed. On the other hand, massive bleeding all over the place might make a fun change to combat. I suppose I'd play it both ways and see how it went.
Sure would be a fast return to swords and other similar striking weapons, I think. It would be an especially great tactic for monks ... and clerics would definitely get the short end of it.