Originally posted by Hellfire
Okay lets follow this logic a moment.
#1 Every civ has access to armies, so who has the unfair advantage
No - only civs which have generated a great leader and chosen to use the leader to create an army have access to armies.
#2 Firaxis introduced armies themselves and they turned out to be unbalancing?
No - Firaxis programmers stated that being able to upgrade units or unload units (same thing as upgrading, as Hades pointed out) would be unbalancing. The army itself is not unbalancing.
#3 instead of removing armies entirely, they "nerf" them (to borrow a term from MMORPGs) so they are so much less useful than they could be.
No - they kept armies in the game because they remain useful - just less useful (read: not unbalancing) than they could be with alternate rule sets.
The frustration with armies is they are so NOT useful because they don't behave like a stack of normal units. They behave like a stack of units with fewer abilities than regular units. Thats so stupid. Either Armies are in with full abilities or they are out entirely. This is just an example of a programmer or designer trying to be cool and failing miserably.
I find armies very useful. I particularly find mixed-unit armies useful. And they shouldn't behave like a stack of normal units - if they did, why would I bother with armies?
Mixed-unit armies are useful for a couple of reasons. The short version (for me) comes down to one particular key point - I expect my army to essentially guarantee a battle win in almost any circumstance. Two swordsmen or two horsemen in an ancient age army just about get you there; 2 knights in the early middle ages will do the same. Adding a third unit doesn't appreciably increase the odds of winning a battle - the odds are so skewed in your favor due to the 8 or more HPs (no chance for defender promotions before death) that adding a third unit might rightly be thought of as overkill (might take expected battle win percentage in a given circumstance from 96% to 98%? not a big gain). Now also take into account that armies can be few and far in between until the ability to build the Military Academy arrives with Military Tradition. I don't want my army to become obsolete quickly -- as may naturally occur as tech progresses -- because I don't know if I'll get another one into well into the Industrial Age.
An example probably helps here: If I have carefully left a space in my early army (loading only 2 horseman, say), I can add a knight to my army as the age of chivalry arrives. The 2-horseman army is an exceedingly powerful ancient age attack unit, and I am pretty comfortable that it is going to take down anything in its way (yes, there are a couple of exceptions

). If I add a third horseman, the relative strength of the army vis-a-vis other ancient age units is largely unchanged - it remains a supremely powerful ancient age monster. But soon the age of chivalry arrives, and with it knights, pikemen and musketmen, together with more Cities (7 pop or higher) and the defensive bonus Cities provide. Had I created a 3-horseman army in the ancient age, the army itself would now be largely obsolete - sure, I could use it against spearmen, and I could use it against units in the open field, but I probably can't confidently use it to "crack open" that City guarded by a tough defender. However, since I only created a 2-horseman army in the ancient age, I can now add a knight to the army. My knight-horseman-horseman army is still very powerful in the Middle Ages - its 12 HPS, led by a 4 attack unit and then two 2 attack units, means it still retains the key capability I desire from my armies - the ability to essentially guarantee a victory in battle. By loading units into armies over time rather than as soon as an army appears, you can successfully extend the useful life of a single army significantly.
A second reason to use mixed unit armies (particularly mixed units across ages) is that armies appear to "share" hitpoints. What does this mean? A 2-swordsman army defends with a "2" defense. Add a musketman to the army, and the army now defends at a "4" defense. With three units overall, the army probably has
at least 12 HPS. If this mixed unit army is attacked by an enemy knight and wins, losing 3 HPs in the process, you're left with a 9 HP army. Guess what? That now-9 hp army defends (for 3 HPS) with a "4" defense. You have successfully leveraged the "2" defense HPs of the swordsman into "4" defense HPs of the musketman. Not bad - and goes a fair way to overcoming the limitation on upgrading units in an army.