Celebrim
Legend
Usually the Weapon Vs Armor chart went out when the DM quickly realized that nobody knows what armor 99% of all monsters are wearing, and even for the ones where it IS stated you had to go read through the MM entry to find it. At that point all but the most detail obsessed DMs tossed it. I think Gygax actually said something about putting them in being a 'mistake'. OTOH I expect he was one that tracked encumbrance and components exactly...
The Weapon Vs Armor chart is one of the best things about 1e.

In 1e, you used a sword, right? Probably a long sword; maybe a two-handed sword but definately a sword. Why? Because they had far and away the best damage of any weapon. It was a no brainer. You used a sword because everything else was so inferior.
Because you weren't using the Weapon Vs. Armor chart. The problem with longswords is that though they do good damage and are quick and nimble against lightly armored targets, they really suck against targets in full plate. What you want against an armored target is a morning star, or a mace. Even a spear is better. What the Weapon Vs. Armor chart did was bring balance and trade offs. It was one of the little used discarded rules of 1e combat that, if you actually took the time to use them, brought combat alive and made it tactical, diverse, and interesting.
It was revolutionary to how I looked at the game when I started using it. First, you are right. The armor of monsters wasn't well documented. I had to look at each monster as I used it in my game and figure out what portion was armor, and what portion was everything else. So, the first thing I did was start giving each monster and AC and what I called the AB (Armor Bonus) by breaking them down logically. But that implied very quickly that a portion of the AB was often Dex, and so that entry went along side each monster as well. For the first time, all of my creatures started having ability scores. I could start comparing their abilities to PC abilities. Some of them acquired Initiative bonuses. Strength was already stated or implied in many cases, so why not fill that in two. Intelligence was listed, so now I had that. Ability checks could now become a standardized mechanic, replacing the hodgepodge of percentile tests and other random improvised mechanics used to judge noncombat things before. I was, right about the time I got overwhelmed by the scale of the work converting the rules in a time when the SRD didn't exist and I didn't own a word processor, in the process of converting the monsters attacks into weapon equivalents and thinking about adding Con scores to monsters to solve the problem in 1e of most monsters presenting too little challenge (post adoption of the UA rules).
They actually weren't that hard to use in practice. The PC's were the biggest headache, but they only carried 3-5 weapons each and didn't change armor all the time. It was a simple matter to simply create a to hit table for each PC. That ended up saving more time than any innovation I'd ever done then or now. Instead of players adding up all their piddly bonuses every attack, they just could report the number throne. I had the target numbers for each AC precomputed. No cross referencing. No addition. One table for each PC. After that, I was resolving combats faster and more fluidly using 'to weapon vs. AC' than I had been before using it.
If 3e didn't have so much additional complexity, I'd bring back to weapon vs. AC modifiers in some form for my homebrew. It's one of the few areas of 1e I really miss. Segmented rounds are another one. Simultaneous declarations and resolutions are another. I've been tempted by all three, but I just think it would make the game too complex. There are trade offs in realism vs. playability.