With the rules for cavalry charges in place, let's discuss its impact.
Charges are definitely a climactic action with this mechanic. The attackers are risking losing their mounts and suffering 1 or 2 unopposed attacks against themselves. However, they can totally break an opponent's position, and if able to pursue, rain down a number of unopposed attacks themselves.
While it seems like charges could make combat trivial to the point of first charge wins, take a look at monster Morale values. It seems like many rolls would come down to both sides succeeding, so it would only be applied in cases where the chances for success can be tilted towards one's favor. Typical values for humanoids range from 7-9, meaning that a failed rolled is not going to be common. Even so, on most bad rolls, the defenders won't be failing by a wide margin. There are a large number of monsters who have 12 morale since they are undead or otherwise non-living. Plus, a defensive line equipped with Pikes will have a morale in the range of 9-11, making them rather safe from cavalry charges.
On the other hand, this option can offer some variety in fights and turn typical encounters on their head. Mass amounts of low HD monsters (or hirelings!) are particularly susceptible to a well timed charge. Morale affects have a much more profound affect; a bless or curse spell could really change the tide of an encounter. Controlling the field of battle also plays an important role; if one side can't defend against a charge then they need to pick broken terrain that won't allow a charge to occur.
I'd love to actually see this put into play during a game session to really push the system. Ultimately, it is only a roll of a few D6s so the true test of the mechanic would be its application during normal adventuring. How often would players use this type of tactic? How often should a DM throw in some goblins on worgs who are willing to charge their enemies? Orcs on horseback? Would horses and mounts actually be worth their corresponding real world value in D&D beyond a convenient method of travel and hauling loot? Could you work in a gigantic mount, like an elephant or something even bigger, and really go on a tear?
One day, I'll get to put this rule into a game and see how often it comes up...