Armies aren't really all that useful. They are like a unit with a ton of hitpoints. The problem with the army you created is that the 1-move riflemen will slow the army to 1-move. Second, armies can only attack once per turn (which sucks if you put Modern Armor in an army... they can hit 3 times normally). In general, three units operating independently are better than an army. However, I have found a use for armies:
The AI will rarely attack an army, because it knows that it will probably lose several units doing it. Therefore, if I invade, the army is used to protect my invading troops, whether it be a large stack of Cavalry, or the slower and even more vulnerable artillery.
Using an army on the attack is... well... no better than using the units in the army independently. Basically, if there is one unit you want dead, the army will make it dead. Armies are NOT invincible juggernaughts. I had an army of 3 veteran knights once that I used to attack a city with a musketman defender (Cavalry was a couple of turns away, and I wanted it NOW!). I killed that musketman, but my army had 1 hp left.
-Arrian