@classical hero: The AI does absolutely know how to use an Army properly. I have seen an AI Army in RaR (they're autogenerated there by a couple of small wonders @30turns) even pillaging correctly.
What is messed up, is how the AI fills Armies: They load one 'Offence' unit in it, and then the formula demands to fill it with units of matching speed. But in c3c, Armies are +1Movement...and that part was forgotten to be updated. If you modd an Army-generating improvement/wonder into the regular game, the AI will join a Horse or Knight first in the early game. They can fill it with Cavalry/ModernArmy later, but since most approaches made the MilAc generating Armies, the AI will start with a Cav - and since there is no 4 movement unit, the AI will never use it (the AI generally uses only completely filled Armies).
In Middle Ages, all Civs can build Swords from start; thus, if England is attacked early on, it will load a Sword (or sometimes a Warrior), and can fill it Horses or Knights later.
In RoR, Carthage has no good move 1 attacker, and the other Civs lack a move 2 unit before Heavy Cavalry. So the starting Armies in RoR aren't used at all for most of the time.
In RaR we have a contemporary Marine-type slow attacker throughout most parts of the game, and the AI loves to build those. Thus Armies composed of a Marine/Tank are much more frequent. And, Armies in RaR only hold 2 units before the Pentagon.