The problem, as I see it, is all cav units, no matter their cost or charge value can kill most of a foot unit (whether spear, pike or sword) on contact (or within a short time after contact). Heavy cav, like the well armored heavy knight units should be able to kill a lot of units on contact due to their armor, mass, and cost. However, even the cheap, unarmored, light cav units kill the same amount of men as the heavily armored, expensive, knight units during the initial charge. I think this is where the actual problem is.

A light unarmored cav unit should not kill as quickly or as many foot units as a heavy armored cav unit.

Certain factions might as well not be included in MP (like the Aztec faction) if certain changes aren't made to some of the cav units. The Aztecs have no cav units available so nobody will take them if they think a player will bring a mostly-cav army.

Proof that cav is overpowered will soon be easily noticed when you find most of your opponents using mostly-cav armies. Just like in MTW and the knight, sword, pavise crossbow armies you saw all the time. The reason you saw those units was because they were the most powerful in the game. Spears have been nerfed since the very first MTW patch. I do not see them being brought back as a powerful anti-cav unit either, due to what Puzz3D insinuated: CA has determined or believes that the majority of the newer players like having strong cav more than balance.

Another problem is CA has never created two sets of unit stats: One for SP and one for MP. Currently, changes to one will almost always adversely effect the other. CA has always been SP focused, so you will be hardpressed to get any changes which will balance MP, but may unbalance SP. SP still rules.

I can almost imagine what started this idea of high-powered cav during the meetings for M2TW:

Someone asked; "What do you think of when you think of medieval warfare?"

"I think of knights in shiney armor charging down their enemies!"

"Very good Mr. President of Marketing"

"I think of balanced armies and spear units impaling horses and bashing in the knights' skulls!"

"Very good Mr. Wise Guy. I hope you enjoy your next job assignment in the mail room."