In a blog post on Fighting Fantasist, the logic behind all weapons doing 1D6 damage in OD&D is argued. I read this post a couple of days ago without thinking about it much, other than "huh, that's an interesting perspective." Today I found a post offering a rebuttal on the blog Elves Ate My Homework.
Obviously combat in tabletop RPGs is an abstraction, and these two different systems are merely different approaches to simulate the same thing. Both are imperfect, yet from a simulationist point of view it's difficult to argue for one vs. the other.
Consider the typical D&D combat with a diverse party of 4 or 5 player characters fighting an approximately equal number of monsters. It's easy to see how the 1D6 for all weapons is an oversimplification here, for the exact reason stated in the rebuttal blog: the Rogue is at a severe disadvantage using a dagger when fighting toe to toe with an orc wielding a bastard sword. Because of the significantly shorter reach, the Rogue is unlikely to be able to get close enough to deal an injurious blow to the orc. The fact that a round represents not just a single attack, but a span of 6 seconds (with the attack roll summarizing your overall success during that time span) means that using the OD&D rules, the Rogue would be able to deal just as much damage as the Orc. Truth be told, I simply don't buy this. Later editions of D&D (including 4th) in which the dagger deals 1D4 and the bastard sword 1D10 seems to approximate a better "average" for how well each combatant does.
Obviously a balancing factor for the Rogue who uses a dagger is going to be sneak attack. Extra damage is applied (and only when using a light blade, in 4th edition) when the Rogue has combat advantage (or is flanking, as is the case in 3.x edition before the term "combat advantage" was succinctly defined) because the enemy is distracted. The "all weapons are equally as lethal" argument in the Fighting Fantasist post is represented in a much different way; namely the sneak attack system shows that when the enemy is distracted, a Rogue can deal massive amounts of damage with an "inferior" weapon such as a dagger. It's clearly not inferior because of it's damage potential; a dagger can easily pierce a vital organ. It's inferior because of it's length, which is made moot if the enemy is worried about something else.
This brings up another important question though: why can't all characters deal "sneak attack" damage, and why can't they use bigger weapons to do so? After all, it represents taking advantage of an opening in the opponent's defenses. I'd buy the argument that an axe is too slow and unwieldy to capitalize on a short opening, but what about a bastard sword which can be thrust forward like a rapier? Is thrusting with a dagger really that much faster than thrusting with a full sized sword? The argument could actually be made that it's slower, as normally a dagger wielder wants to be outside of his opponent's weapon's reach, and to take advantage of a hole in their defense requires stepping forward and then thrusting. A bastard sword wielded against another sword user wouldn't require the forward lunge to be quickly thrust at the weak point. Clearly this system isn't an accurate representation of reality either.
Consider further the issue of "surprise attacks." You ambush someone in an ally who isn't ready for you, for example. Assuming they're unarmed and possibly unarmored, a solid blow in a vital area from any type of deadly weapon is going to be lethal. This is consistent with the Fighting Fantasist argument for all 1D6 weapons. But what about a blow that's not so expertly landed? Say, the opponent dodges at the last second and you end up hitting the shoulder. A dagger might hurt like a bitch, but an axe will likely take off the arm.
I guess the point is that there are too many variables in real life to sum up when and if a given weapon is going to have an advantage. The more accurately simulationist a game strives to be, the more it gets bogged down by complicated rules that constantly need to be looked up. After all, you'd also have to factor in what types of armor the opponent wears, and will that give certain weapons an advantage? Halberds are designed to pierce heavy armor, thus this weapon should be nearly as lethal to heavy armor wearers as opponents in light or no armor (in comparison to other types of weapons, of course). Conversely, an axe's lethality would be severely hampered by heavy armor. It's not accurate to target weak points, and the edge that's designed to cut will have little effect on steel (turning the axe into an inefficient bludgeoning weapon, essentially). Could you imagine how infinitely complicated a game would be if it strove to keep track of these differences in not only armor and weapons, but all of the combinations of weapon types vs armor types?
In the end, both systems (fixed die and variable weapon die) are situationally appropriate, and both suffice to summarize combat as an abstraction rather than as a simulation. One thing that I disliked about 3.x edition was that it strove for a relatively high degree of simulationism, but is still at its core an abstract system. I've never played an edition of D&D earlier than 3rd, but from what I understand 4e streamlined D&D such that it resembles earlier editions more than 3rd edition. Both 4e and OD&D openly acknowledge that they're not simulationist, yet partially through historical accident 4e retains the variable weapon die system. I think I like that best. It provides diversity and does well to make weapon choices meaningful, but at the same time it doesn't overly complicate things (a given PC is probably going to use one or two different weapons). One unfortunate side effect of this (which is absent in OD&D's single weapon die system) is that certain weapon choices are obviously sub-par. Character concepts designed around a certain weapon because that weapon "cool" may be penalized in a variable die system, whereas in a single die system all concepts are equally as viable as any other.