Why AC instead of physical resistance?

[pampl]

Another pie-in-the-sky game mechanics question from pampl I understand the point of having a number which represents completely avoiding an attack, but I'm curious about why that number should be A) tied to physical damage reduction and B) partially be a function of armor thickness. The alternatives as I understand them are:
A) The system fizzix implemented (IIRC), and that crawl uses, where evasion and damage absorption are separated. This is nice for simulation reasons because it intuitively makes sense the two are separated, and nice for gameplay reasons because it lets you have nimble rogues and mages who can dodge attacks but aren't good at reducing the damage when they're hit. It also lets you treat spell and breath weapon accuracy like other projectiles without having their damage reduced by armor. The downside is that it still treats physical damage differently than all the other damage, which arguably doesn't make sense in the world of Angband where physical damage is relatively unimportant (though still the most common)
B) Split evasion and absorption as above but treat absorption like any other resistance. This is more consistent, but if the resistance is on all armor then it becomes meaningless, and if it's not on all armor than some non-magical armor becomes literally useless. There's also the problem of someone just wearing plate shoes (for example) to get physical resistance then wearing light armor everywhere else. It's also less granular than AC is for damage absorption - it would eliminate the differences between, say, chain and plate or robes and leather.
C) Treat physical damage sort of like acid and have it hit a random slot. If that slot has the HEAVY_ARMOR flag then the damage is reduced by 50%. Avoids the shoes problem, and makes more intuitive sense, but still isn't granular so it collapses the differences within heavy armor and non-heavy. That stuff all becomes just flavor. Also it's a hidden rule that players wouldn't immediately pick up on unless we add new combat text ("The orc hits you in the foot!")
D) Use additive resistances and make physical resistance a category like any other. I'm not a big fan of additive resistances, but this is a case where it seems like the best choice. It offers granular distinctions between armor, it's consistent between physical and other damage, and it's an immediately understandable game mechanic. Is there a problem with it I'm missing?

PS: this is a theoretical discussion; I'm not trying to demand massive changes in Angband.

[Derakon]

Additive resistance is exactly what armor currently does. You get damage reduction linearly proportional to your AC up to an AC of 250 and damage reduction of 60%.

This isn't messaged anywhere in the game, mind you.

[PowerDiver]

Because that's how it was done in D&D when the gameplay mechanics were mostly copied. The reasons it was done that way in D&D were presumably simplicity, fewer dice rolls, and improved player survivability compared to more realistic systems where a lucky blow to the head essentially kills the char.

[pampl]

Additive resistance is exactly what armor currently does. You get damage reduction linearly proportional to your AC up to an AC of 250 and damage reduction of 60%.

This isn't messaged anywhere in the game, mind you.

It kind of is, but it isn't really consistent with other resistances. It offers no protection from side effects, it's added to temp resistances (bless, shield) instead of multiplied, and temp resistances don't let you exceed the maximum. And, like you said, it isn't displayed as a resistance in the same way the others are. It's a resistance in the same way FF is gravity resistance.

If you were to make it more consistent with other resistances, what would you do? And do you think it would benefit in any way from being made more consistent or is physical damage different enough that it should be handled in this substantially different fashion?

[Derakon]

If I were to make AC consistent with magical damage sources, it would be by moving magical damage to an AC-like system. By making the system more fine-grained, you make it harder to have an obvious decision, which in turn makes the whole equipment-optimization process much more difficult. Ideally it should be nigh-impossible to say what the best equipment loadout is from a given selection.

[pampl]

Because that's how it was done in D&D when the gameplay mechanics were mostly copied. The reasons it was done that way in D&D were presumably simplicity, fewer dice rolls, and improved player survivability compared to more realistic systems where a lucky blow to the head essentially kills the char.

Yeah, but I'm trying to think about it independent of things like inertia or original inspiration. If we were operating in a vacuum trying to build a game about a novice adventurer growing vastly more powerful then killing Morgoth, how would Angband's system compare to alternatives? Is AC too simplistic given that we have the power to track a million different variables? Player survivability still holds water as a reason, I think; Angband is a long game with epic (read: many turn) battles so having a small chance each turn of losing an arm or being decapitated or whatever wouldn't work so well.

[pampl]

If I were to make AC consistent with magical damage sources, it would be by moving magical damage to an AC-like system. By making the system more fine-grained, you make it harder to have an obvious decision, which in turn makes the whole equipment-optimization process much more difficult. Ideally it should be nigh-impossible to say what the best equipment loadout is from a given selection.

Have you tried Torchlight? It has fewer resistances but they work the same as armor does and the large number of modifiers on items means it's very hard to choose between two pieces of equipment. IMO it actually hurts strategy, though; there are so many things to consider that people just choose randomly or use very simple heuristics (whichever adds the most to my primary stat, whichever adds the most to my lowest resistance, etc). Just because Torchlight failed (IMO) in that respect doesn't mean all systems with complex equipment decisions will fail, of course, but I don't know what the magic element is that makes the difference between a hard choice that players spend time thinking about and a hard choice that players ignore in favor of simpler choices.

[PowerDiver]

Yeah, but I'm trying to think about it independent of things like inertia or original inspiration.

Then you are asking more "how should it be" rather than "why is it". I guess your title could be taken as you meant it, but my useless answer was answering the question I thought you were asking.

There was a recent discussion about changing from AC to separate absorption + evasion. How did you manage to miss that? The problem is that it's a huge amount of work, to be followed be a very long time to rebalance things, and who knows whether the final product would be noticeably better than what we have.