Light Cav has several great uses.

First, since it is fast, it is about the only way you will be able to chase missile cav with any effect.

Second, it is terriffic against the rear of an enemy formation as it can get into position to charge easily. After they charge, pull them back IMMEDIATELY. This not only avoids having them sit there to take casualties, it also allows them to set up another charge. Scot Border horses are fantastic for this.

Third, Light cav is fast enough to move around and threaten areas. If you have troops behind the enemy, they take a morale hit. They will also often come after your fast, cheap horse troops with something expensive. It is not a bad thing to have four light units running around if the enemie's four Generals are preoccupied chasing them.

In all, I tend to find light cav more useful than heavy cav in most fights.