Knowledge skills represent knowledge (no!), of the subject in question.
What can you do with this skill?
Identify monsters and their special powers or vulnerabilities: This right here is why some players take this skill. It either provides justification for why a character would know something a player does, or it actually helps when you make that roll and learn something useful. There is an issue with that whole common vs. rare monster thing, where the CR and the commonality of the monster set the DC to know something useful. I don’t think that it’s unfair to assume that the powers and vulnerabilities of certain monsters are really well known.
Who doesn’t know these things :
- Vampires and the sun don’t mix (besides Stephenie Meyer)?
- Zombies like to eat living flesh, perhaps especially brains.
- Goblins are physically weak pyromaniacs.
- Dragons will kill you, and you may as well not run, ‘cause you’ll just die tired.
As DM, you need to make common knowledge common. Don’t waste your players’ time making them roll for stuff that everyone should know, and don’t make up reasons for why they wouldn’t know things. If the vampires in your campaign world are more or less just like classical vampires, don’t make your players roll for that knowledge. It’s a serious drag on fun and play time to act like you don’t know something so culturally ingrained. The whole point of using archetypal monsters is that everyone from a certain culture will recognize them for what they are. Don’t be a dick and defeat the purpose of the thing.
On the other hand, if certain monsters in your campaign world are rare, unique (or nearly so), or live in inhospitable areas and are rarely encountered, that’s a different story. Maybe your PCs have heard of the Dragon of the Puce Hills. They know that it’s rumored to have strange powers. When they encounter this wingless horror, then they can make a roll to recognize it as a Crag Linnorm. If they fail, and didn’t do any research, they hey, they’re not going to automatically lose the encounter (whatever that means…), but they will have to deal with the consequences of running off half-cocked. Maybe the wyrm likes to make deals, but they choose to attack it. Maybe they try to diplomatically interact with a crazed beast. Maybe the damn thing likes strawberries and will let them go if they give it some (and what kind of horrific beast doesn’t like strawberries? The worst kind.) Do you need to give them this information for free? Not necessarily. You just need to make it available to them, somehow. That puts the onus on the players to figure out what they want to do.
Please note that this applies to monster information that is not absolutely vital to the player’s chances for success. If they have to face a beast with invulnerability to all but cute kitteh pictures, and you’ve made it impossible to survive this inevitable battle any other way, then you’re a bad DM if you don’t give them this information. If they have to know, you have to tell them, and also give them a way to get kitteh pictures.
Know subject material not directly related to fighting things: Here’s where this skill is overused/underused/wasted. And there is a legitimate question as to the value of the skill here.
Information is the only commodity that you give to the players. The game is literally a bunch of people sitting around telling a communal story, and pretending to be the main characters in said story. The DM talks, players listen and respond. It’s entirely an information exchange.
So it is absolutely incumbent on the DM, then, to insure that all of the information necessary for the game is made available to the players. This could be construed as an argument that the knowledge skills are useless; a DM should just tell the players what they need to know, and whether that information is trivial, of moderate use, or vitally important doesn’t matter. This argument has merit, in that it certainly won’t make your game worse, or make your players unhappy. In Old School gaming, you might have just listed a secondary skill, or worked with the DM to determine what your character knows, and there’s absolutely nothing wrong with that. It defines a character and speeds up gameplay, and can definitely improve the experience of playing.
But there is also merit in the idea that the skill helps determine what the player knows. This method spells out a clear system for determining what your character knows about a given topic. The trick here is to see the numbers for what they are; to define them for your players. Because the game models reality best at levels 1~6 or so (I speak specifically of D20 versions of D&D here), it’s important to note that someone with a single rank in a skill is at least equal to a trained journeyman in their field. They may not have a lot of natural talent, but with the right tools and assistance one rank goes a long way. At three ranks, a character is a seasoned pro, and at five ranks, you’re dealing with a master. More ranks than that are just gravy.
Characters above 6thlevel are more or less superhuman. I have no issue with that; it’s a lot of fun to be able to headbutt a ghoul to oblivion while drinking some ale. But it’s important in how this relates to the skill system’s inner workings. The point is this – when someone has 5 ranks in Knowledge (planes), you don’t roll to see if they know that demons have fire resistance, you just tell them. They should know that already. What you roll for is to see if they know that the babau demons currently trying to kill them can always see the invisible. That’s a piece of moderately useful information that won’t kill them if they don’t know it. The babau might kill them; but they won’t have lost against the demonic assassin simply because they didn’t know.
Give the players information that they should know without a roll, and err on the side of generosity. A podiatrist may not be able to identify someone’s pancreatic cancer with a short interview and a glance, but they’ll know enough to know that something’s wrong, it’s out of their discipline, and they’d probably have a good idea of who to get in touch with.
The random nature of Knowledge rolls can be problematic; the easy way to deal with that is to simply take 10. This gives your character a basic pool of knowledge and avoids the annoying bad roll which leads to not knowing the answer to a very simple and easy question. If they don’t know something by taking 10, you could always let them roll anyway. Maybe they need to reflect for a second or two to remember some obscure thing.
I would guess that some will always prefer to use player skill to resolve this sort of thing, or to just tell players the information that isn’t vital rather than waste time rolling. There’s nothing at all wrong with that. I like the idea of the skill, so I use it.
Final Analysis: I feel that Knowledge skills can be fun, and improve the game. You have to give the players some leeway with these skills; it’s always a drag to feel like you’ve taken a wasted skill. But being able to roll for a chance to know something about the weird threat you’re facing is always nice. I also feel that a character should be able to apply Knowledge skills to gain tactical advantage, with some small accompanying bonuses (usually competence). Your wizard has Knowledge (engineering), you say? Then you may ignore 5 points of hardness when you attempt to cast the lightning bolt to collapse the cavern onto your foe, because you know where to strike. It works in the nebulous regions of play, where there isn’t strict definition but you don’t want to turn something into a free for all. I invite comments on this; I feel like there’s more to hash out on this topic.