One of the problems with hard caps (leaving aside unrealistic arbitrariness for a minute) is that it results in a situation in which it is basically impossible to win unless you have the most up to date units. This may not be a bad thing, you would think, but then consider situations such as World War Two in Russia. The Soviets did not have the technological upper foot, but were still able to win through sheer weight of numbers. If there was a hard cap that would actually have an effect in diminishing stacks (meaning that it would be a relatively low cap, or it wouldn't have any effect on the game), then numerical superiority would be virtually unattainable, and so victory would be virtually unattainable unless you have the most up to date units.
Also consider the fact that if you have a large city, holding its maximum 20 units, it is outnumbered simply by two adjacent tiles being filled by the enemy; 24 units. City defence would become near impossible.
Actually I have seen some games do this hard cap and it works rather well. You can still use power in numbers. For instance, in the final example you gave, you would most likely have your 20 units in the city along with X stack(s) of 12 units roaming the outer 8 tiles in a defensive positions. So then with 1 extra stack you now outnumber them 32 to 24. In games that do this, you will quickly learn to guard a tile with more units than can fit on the tile itself.
Windsor said:
But would it really change things? Except making it more tedious. And instead of moving all troops at the same time, you'll have to move several stacks. When your attacking a city you'll need to go through a bunch of stacks to find the right unit.
This game could stand to use some tedium in warfare. Warfare is WAY too streamlined. What getting rid of SoD's would do is offer variety through tactics. This is not a very tactical game, thus it is not a military game. The only real "tactics involved are shoddy at best. (i.e. As spoken before about rilfes/cannons vs. cavalry/airship vs. paratroopers/gunships.)
What do you wanna build your SoD out of is your "tactic". Or you could say that your "tactic" is to hit target A first, then B, etc. These are more generalized plans though than tactics.
I would like to hear what options you see limiting stacks would open up.
I honestly think that opening up a cap on tiles will teach the AI how to war more efficiently for one thing. But on the grande scale it would open up the ability to add more tactical options to the game. For instance specializing against a particular type of warfare is suddenly much more beneficial if you are only allowed 12 units on a tile. Which 12 units do you use if you are facing knights, muskets, and maces primarily? Your stack best suited for knight and musket warfare (Probably pikes/knights) could run into knights and crossbows (anti-melee stack.) Also, where is your artillery going to go? In a stack by itself? 2 in a stack? 4? How would you choose to guard it knowing that is the first stack your enemy will want to destroy. You will most likely surround your artillery with 12 unit stacks.
It allows the defender and the attacker the ability to outmanuever their opponent. Pick the weakspot and exploit it. As most people will probably still run "stacks" but it will be a "stack" spread over 5 or 6 tiles. Of which you can pick where to hit the stack.
What would be possible with stacks-limits?
Stacks that have an exploitable weakness.
From this, it could branch off into more depth in warfare. But currently there is no "weak spot" in a stack due to the fact the best defender of your entire ARMY always defends.