It's exactly what happened in real life as well. Forward facing armies were dominant for quite a long time as a military tactic, but were weak from the flank. This led to cavalry charges (when possible) on the enemies flanks, and destroyed them because they couldn't turn to face the incoming threat (it was slow to turn and face them, and then it left them vulnerable to the spearhead of your own infantry).
Cavalry was very powerful for a while until armies started using pikes in their formations (lots and lots and lots of pikes). Not to mention crossbows being able to be picked up by anyone with little training and being useful (en masse) on the battlefield.
I wish for two things in c-rpg:
1) Infantry that was able to fight in formations (even loose, semi-formations) - this would require better communication in game, and game play mechanics to support it (i.e. being able to have people march at the same speed).
2) Braceable pikes/spears/lances that if you run into them, you get hurt. If you're riding a horse and run into them, you get skewered. To expand on this, any weapon could be run into by anyone and cause damage (the weapon has an ability to inflict damage even if not being swung). Also, horses should be able to get "stuck" on pikes, and basically stop their forward movement immediately (instead of the corpse turning into a ghost and flying through the pike). Also, then I would like to see the bodies of horses be able to cause damage after they die. Same with people, if someone jumps off a roof and lands on you, it's probably going to hurt them, but it should hurt you too if they fall on you.