Quote Originally Posted by Goofball
But the opposite can also be said to be true. If some sort of RPS system isn't used, then you will certainly have armies compased of the same unit all the time.
That's a danger, especially in SP where the budget constraint typically does not stop you maxing out on elites. Some mods, or just self-discipline, can overcome it, however.

But I am not sure RPS is necessary to obtain variety of army composition, or interesting tactics. If you think about real military operations, they are not necessarily RPS. For example, for a long time, a tank was the best way to kill another tank. The best way to get air superiority is to have better or more fighter aircraft etc. But you don't see armies composed just of tanks or fighter aircraft. It's not that different arms behave in an RPS way, but they have different comparative advantages or functions.

For example, in the TW period it's not that missiles were good against spears per se (as one RPS interpretation of STW implies: archers beat spears, spears beat cav, cav beat archers). Rather it is just they were useful for softening up an enemy and lowering morale so that your melee units can win with less loss. The STW tutorial, with an archer unit against an attacking spear unit illustrated the edge from a little preparatory missile fire rather vividly. But in general, missiles were pretty useless at holding ground.

Cavalry was good for flanking, due to its speed; for pursuit; for scouting; and for fighting other cavalry. And for shock combat against inferior infantry.

Heavy infantry was good for holding ground and taking well defended ground. In the Medieval period lower quality infantry (spears in TW) might only be useful for defense; higher quality stuff (swords in TW) might be necessary for taking the offense.

I don't think any of the above is RPS per se. I guess it's combined arms - typically having all three arms would generate synergies. Most of it, probably all of it, is pretty well captured by TW - which is why, despite its broader appeal, it attracts fans of historical wargamers like me.