Those paragons of us-against-the-world ruthlessness did it:
"Elmo, pick some good halbardier and crossbowmen. Quarrels to be poisoned."
- The Captain, in Glen Cook's The Black Company
That's just one example from those books - poisoned dart-shooters show up in Shadows Linger, too, and arrow poison in the Books of the South.
But what about in games?
I've heard a lot of arguments against it, some of which Jim Ward discusses here, while endorsing its use:
"Then there is the poison on the dagger trick, which every judge is always trying
to stop. I have been told that poisons evaporate, poisons exposed to the air lose
their effectiveness, or the most used of all, in your area there is no poison strong
enough to kill the things you want. I suggest to all you players and especially the
magic users that can use only daggers, that any amount of money and effort
spent in the procuring of a really effective poison is worth it. I spent over 90,000
gold and haven't regretted a copper piece of it."
- James M. Ward, "NOTES FROM A SEMI-SUCCESSFUL D&D PLAYER," The Dragon #13
My players seems to agree with him. One of the two scouts has every single one of his arrows poisoned. Venom has come up in discussions of tactics. The real death-dealers skip it, only because they mostly don't need it to kill the monsters they most often fight, making it less cost effective.
But old-school AD&D pretty much stomped on poison use, and presented those objections raised in Mr. Ward's piece.
Still more can be found in the 1st edition PHB, 1st edition DMG, and even later D&D books like the 3.0 DMG. While the 1st edition DMG is a bit more generous than the others - it says assassins use poison "just as any other character classes" - but it still limits it a lot. It's got a lot of the same limitations on poison.
These are notably absent in GURPS Dungeon Fantasy 1, which makes poison a straight-up option on the equipment list, and the only benefit from being skilled in Poisons skill is the ability to use poison more effectively. Otherwise, it's assumed it is ready (and fairly straightforward) to use. Rolemaster had plenty of nasty, nasty things you could use on weapons, although mostly people were too busy lopping off limbs to pay money for poisons in my games.
But what about the objections in the AD&D books?
Some I don't buy, or buy only partly:
The poison will dry up and/or lose its effectiveness. Sure, maybe it will. So why not store it in breakable containers? If you say there aren't easy to transport but also easy to break containers, well, flaming oil is right out, too, potentially. So are pots of green slime and other sneaky tactics. Or just put it on before you enter the dungeon and stab something pretty quickly.
You can also take a leaf from The Wild Geese, and put breakable containers of venom (cyanide, in the movie) behind arrowheads. There is some suggestion that's how venom-using hunters do it. Maybe Gary Gygax was right and it'll dry out and won't stick to blades . . . but mixing up a sticky version might help, or alternate delivery methods can be used.
People will attack you. Seriously? The whole "hue and cry for the watch and then attack" bit is pretty funny. Joe the Blacksmith will attack the hulking barbarian because he was a poisoned sword? Yeah . . . that seems pretty smart. Your hirelings will jump you in the dungeon? Why?
The assassins guild will attack you? Why again? Competition? They want people to know if you die from poisoned wounds, they did it? Again, I can see them limiting sales to members, assuming there is even an assassin's guild, but what about the alchemist's guild? They might have something you can use as poison. Even merely biological agents - things that will putrefy wounds - or agents that will poison food - are useful against monsters. Does anyone mind if you put out poison food to kill basilisks or wipe out giant rats?
People will besmirch your honor. You have me here. They will, and they should, if you're using it in fair fights. I bet people don't describe "exterminating orcs" as a fair fight issue, or killing umber hulks as an issue of honor. But maybe they do, and you're sensitive to that. Well, honor has costs as well as benefits. Being a murder hobo does sometimes have advantages. Even more so, I bet they'd have an issue with the way you surprise attacked the orcs with a rank of hirelings behind you, the way you call those hirelings "meatshields," or the way you throw flaming oil and fireballs into rooms full of bandits because it's better than facing them one-on-one, or slit throats of Sleep spell victims, or use magical chemical agents (cloudkill in D&D, stench in GURPS). Look, most adventurers don't fight fair in the first place, so poison isn't a fairness issue. It's a social acceptance issue. You might be limited in appropriate targets, but I bet no one will complain if you use poison on predatory monsters.
You might cut yourself with your own weapon. Does this happen a lot? I mean, are you normally assumed to be slicing yourself with your blade on a regular basis? I'm pretty clumsy and I slice myself with my kitchen knife about, oh, once or twice a year. But my Broadsword-18 warrior might poison himself? My 9th level lord is has a 5% chance of poisoning himself (that's the 3.0 rule)? I just have trouble with this. Sure, blow a roll badly enough and hit yourself and you have a poison weapon? Sucks to be you. But it shouldn't be more dangerous to use than, say, a flaming sword or a vorpal blade.
It's a dangerous substance, but so are a lot of substances. Plus an insinuative poison - one that needs an open wound - isn't likely to also be a contact agent, so it's not necessarily automatically unsafe to get a little on you (assuming you wipe it off). Contact venom is more dangerous, but also less likely for use - against furry or scaled monsters, or armored men, contact venom isn't so helpful. Gas or blood agents are more easily delivered, and while gas is tricky (too easy to expose yourself to it) blood agents should be a little safer to handle with the usual adventurer gear of heavy gloves and suits of armor.
You need to know how to use it. I buy this, partially. You do need to know how to use it effectively. If you're just putting it on yourself with no expertise, you're likely to either use too much (and waste it), too little (and waste it), or put it on incorrectly (minimizing the effectiveness of the dose). The more readily available pre-measured doses and simple delivery systems are, the less of an issue this needs to be. And if magical elixirs come in perfectly pre-measured doses, it's not unreasonable venoms wouldn't come that way, too.
It's not fun/it makes the game too easy/it takes away from assassins. Basically the meta-game issues. The not fun part I sort-of get. Shoot it, it's going to die from the ounces of cobra venom you just delivered, so close the door and slink off for an hour and come back. Not the most fun.
The others, no so much. Too easy? Sure, if you have a ready supply of cheap save-or-die venom. So don't do that. The best venom in the book in Dungeon Fantasy 1 that is close to "save-or-die," Bladeblack, costs $1000 a dose - that's the entire starting funds for a PC for one dose. You might get some and use it, but not without some thought. The weakest is $20 a dose, and it's not a bad deal except it does very little damage and it's easy to resist. Worth adding but it's no game breaker. The "it takes away from assassins" bit is a bit more fair - but DF deals with this by making assassins more effective with venom. They can use their Poisons skill to put multiple doses on a single weapon so they take effect all at once. That alone makes it worth more - $80 worth of "Monster Drool" can hurt even a tough monster with strong resistance - and gives the assassin an edge. Not only that, but a lack of instant save-or-die venoms plus stronger monsters (in other words the important targets) means that the ones you most want to poison (dragons, et al) are the hardest to dispatch with poison.
The argument for poisons is pretty simple:
Hunting monsters is dangerous, you need every edge you can get. Yes, it is, and you do. If it's possible to get poison, why not use it? If you can get an effective venom that will hurt monsters you fight and aid your chances of success, and your character doesn't have some limitation that prevents it (Code of Honor, a Vow, or say, being a Paladin), think about putting some money into poisons. Heck, Jim Ward did it. He's pretty old school, isn't he?