For my playing style Cavalry in S2 are useful but only in a specific sense, used to mop up the mess and as a mobile and quick way to turn the enemy army in the direction I wish...My Cavalry are the carrots dangling in front of the enemies noses leading them where I wish on the battle field..
This and the fact that enemy horse (generals primarily) love to attack other horse I am often able to use my Cav to draw out the generals and finish them off early in the battle..
With a lot of trial and error I have found I do not need more than my general and 2 light Cav max to accomplish these things and rarely build yari or higher Cav unless the buildings already exist when I take over a province..

This still pains me as after years of playing medieval I had come to love the achievement of building some truly deadly heavy cavalry but alas for my total war style I do not need them in S2..