Bear with me: this post is going to of necessity assume familiarity with Chainmail and OD&D...and it will rehash and act as further defense for a position I've already defended.
There's been a HUGE amount of discussion recently--some of it initiated by yours truly--about the use of Chainmail with OD&D, "as intended."
Now, luminaries of those early days (Mike Mornard in particular) have come out to say that the original campaigns never used Chainmail, that the "Alternate System," which became the standard d20-based combat of later editions, was always the system of choice. Still, grognards are grognards, and we are scholars of the games we love. This leads to a lot of debate about just how to use Chainmail with OD&D, as it's not 100% clear in the rule books.
Personally, I've noted a lot of these discussions try to shoehorn the whole thing into one of the three combat systems in Chainmail (the "Troop Type," as I call it, the Man-to-Man, and the Fantasy Combat systems). This, I believe, is a mistake, as the original rule books are pretty clear that in general in OD&D, you're supposed to use whatever works, whenever it works. There's no reason this shouldn't translate into the use of Chainmail systems as well. If Troop Types work better and faster than Man Combat, use Troop Types.
Which brings me to another (I think) common mistake: just because the Troop Type system in Chainmail is originally designed for 20:1 play does not mean it cannot work for 1:1 scale in regular OD&D play. A third-level Fighting Man, for example, counts as 3 "men." Looking at the Light Foot entry in Chainmail, they gain 1 die per 3 men against Heavy Foot. All that remains is for the DM to determine how a character attacks and defends (Light, Heavy, Armored).
Let's say for example that your Fifth Level Fighting Man, in Chain armor and shield with a long sword, is classified as Heavy Foot. He's fighting nine Goblins, classified as Light Foot (each having only one hit die and thus each counting as one "man" type). Chainmail tells us Heavy Foot vs. Light Foot rolls 1 die per man, with a 5 or better killing. The Fighting Man, being fifth level, couts as five men. He rolls five dice, and each 5 or 6 that comes up is a hit against one of the goblins, thus dealing 1d6 damage.
The goblins, on the other hand, consult Light Foot vs. Heavy Foot--they roll 1 die per 3 men with a 6 killing. The entire group of goblins rolls 3 dice, with any 6's that come up equalling a hit.
Not counting damage, you're looking at two dice rolls here to resolve a round of combat--one for the Fighting Man and one for every goblin.
Now, let's examine the same thing with the Man to Man rules.
In this case, the Fighting Man gains one attack per hit die--five attacks. The goblins each get one--nine attacks, total. Since each attack is 2d6 compared to a chart of weapon type vs. armor, each attack must be rolled separately. So instead of two die rolls to adjudicate the entire round of combat, we're now looking at a whopping fourteen die rolls, each resolved separately. Granted, once you realize that the fighter against the goblins (clad, let's say, in leather armor with spears) needs a flat result of "8" on 2d6 to hit, you'll know quickly which attacks succeed and which fail. Likewise, the goblins need a 10 to hit the Fighting Man, so they can resolve pretty quickly. But still, it's clearly faster just to throw a handful of d6's and count 5's than it is to roll 14 attacks, checking each one against a target number.
On the other hand, as fast as it is, it's not very dramatic and doesn't very well represent the back-and-forth sway of fantasy combat. It's these situations in which you want to use the Man Combat system. When Conan, alone, faces down with a ravenous carnivorous ape (score it as an ogre; what the heck, right?) in the jungle, or when Arthur and Mordred square off for that final, fateful "Father, let us embrace at last!" battle at Camlann, you want this back and forth. You want the detail and drama. It's anticlimactic if it's over too quickly.
On the other hand, when Aragorn and the Fellowship cleave through a horde of orcs to try and save Boromir as he protects the hobbits, it'd just take all damn day to run rolling every attack.
When (sticking with Tolkien) Bard takes a shot at Smaug with his lucky Black Arrow, that's the Fantasy Combat Table (which, if you know Chainmail, is self-explanatory).
So that's my rationale for mixing systems. Balance between speed of play and drama.
Now, onto my next quibble with a lot of the discussion: there is far too much put into the level of detail in some folks' readings. When you get into breaking down the logarithms and mathematics of OD&D, there's a problem. It's an interesting exercise, don't get me wrong, and quite fascinating to read. And I'm sure if someone is really good with math, it's eminently workable. But OD&D was designed to be simple, and for most people, math outside of dice rolling and single-digit addition and subtraction isn't simple.
Likewise, I think it goes against the spirit of OD&D to champion the idea that troop types are denoted by situational factors such as tactical formation, strategy, and conditions. What DM wants to re-classify every PC and monster every battle based on what they do this time? Can you imagine the chart references and fiddling that would need to be done constantly? Not to mention, classifying troops based solely on weapon and armor matches up perfectly with the man to man system, a fact which is essential if you're going to seamlessly switch between the two.
OD&D needs to be simple, open, and fast, not complex and robust. Just my opinion. With apologies to Shane Hensley and Pinnacle, OD&D should be Fast, Furious, and Fun!
For anyone who is interested, I've done a PDF booklet of my rules for utilizing Chainmail combat with OD&D. It can be found here:
As a final note, I am in no way denigrating those folks (greater minds than me, I assure you) who have done these detailed breakdowns of the mathematics and probabilities involved, and injected impressive amounts of detail into the basics I and others have laid out. It's amazing work, and for a certain style of play I think it's absolutely fantastic. I'll just always champion the "Simpler is [nearly] always better" school of gaming.