Because if I have a limit of, say, 5 units per stack I can make:
Assault stack - 5 swordsmen
Heavy assault - 4 swordsmen, 1 general
Artillery battery - 4 arty, 1 pikeman
Heavy Cavalry - 4 cavalry, 1 general (problems with movement, though)
Mixed force - 1 general, 1 pikeman, 2 swordsmen, 1 arty
Combat Engineers - 2 pikeman, 3 workers
Given the ability to make a stack of 5, I can tailor those stacks to perform various actions.
If a mixed stack with 2 or more cavalry could move at cavalry speed, then I could also make:
Flying arty - 1 general, 2 cavalry, 2 arty.
With 1UpT there's no way to choose or customize your forces. With infinite UpT there's no need to choose. With a limited count you can choose the best mix for the job at hand without getting bogged down by the simple task of trying to get one unit past another.
Excellent point. With the Xupt functionality, you actually have to carefully plan out the stack composition according to battlefield needs.
If you must go with 1upt, change the conception from individual units (tactical) to army groups (strategic). Army groups defined as a collection of individual units where each newly incorporated unit enhances the armies abilities/traits. You could go with two different options for mechanics.
Option A (the simple version) is each unit just gives bonuses to the overall army group and all units take damage equally as they share the armies health points.
1. additional infantry units increase an armies health points. Thus, a large number of infantry will be more difficult to eliminate.
2. cavalry will increase the speed of an army and could provide a significant flanking bonus (more damage) to the army who possesses a larger cavalry wing.
3. engineer units would increase defensive bonuses (mitigates damage)
4. archers provide ranged attack capabilities; more archers equals greater ranged damage
5. siege units provide both siege and attack bonuses
Option B (the complex version), health is replaced by actual numbers of units, each fully healthy unit that you move into an army group would represent a certain amount of troops, and a battle simulation would be run where each group of units in the army act independently and suffer attrition according to the battle they face.
Battle lay-out
The game screen would switch to an animated view of a simulated battle.
The armies would be laid out, more or less, with cavalry on the wings, 3 blocks of infantry in the center (blocks of potentially equal numbers for both sides, with extra infantry designated to the rear as reserves), and archers in the rear.
Battle lay-out would be affected by the terrain within which it is fought depending upon which options the defender would choose to use.
For example, rivers and mountains could permit the defender the option to limit the blocks of infantry to equal numbers on both sides (to nullify the advantage of numbers), and also create only a single wing of battle for cavalry units and would therefore would lessen flanking/encirclement/retreating damage as described below. Hills could increase archer range and damage, and forests could enhance the ease of ability to withdraw without suffering catastrophic slaughter (defenders melting into the woods).
Battles would be fought in rounds, with each round causing partial losses to each individual section of the army.
As battle commences, the two lines would start to move towards each other, archers/ranged units would fire upon the infantry, weakening the infantry. Next, the two cavalry wings would face each other attemtping to drive-off the opposing cavalry, simultaneously the infantry would meet and damage each other.
Each round, as the infantry take damage the reserves would auto-fill the losses.
As the battle progressives, certain conditions could develop:
1. as an army loses all its cavalry, it would suffer catastrophic losses to its archers/ranged and then subsequently suffer severe flanking damage.
2. If an army begins to lose too many infantry and its reserves are exhausted then it would also suffer increased flanking damage as it becomes gradually encircled by opposing infantry.
3. If an army loses all its infantry, than its corresponding cavalry would simply run away and retreat.
4. At the end of each round, the human commander could order a retreat of certain units. He could call off his cavalry, or retreat his archers, or even retreat his main infantry line.
As he retreats his infantry, however, he could suffer significant slaughter casualties if his line has been compromised. The slaughter factor would be mitigated by an orderly retreat of an unbroken line of veteraned units or by the presence of forests.
Hence, if a commander began to see his cavalry losing, he could withdraw his archers to prevent them from being annihilated, or even withdraw his cavalry to prevent their extinction. This would endanger his infantry of course, but he could still do it, and even begin an orderly retreat of his main line.
This sort of battle mechanic would encourage players to really, finally, construct balanced, well-thought out armies.
So, basically, the human would simply watch the animated battle unfold and could intervene only in an attempt to retreat portions of his army.
With Firaxis taking on the Strategic/Tactical scope of Xcom, they could easily learn from their experience there and incorporate a similar idea into future Civ releases.