I mostly use heavy cavaly just for breaking enemy infantry formations also. They are more effective than light cav at this purpose due to their heavier impact.

Contrary to what someone else said above, the rider type cavalry can be very efficient on the battlefield. The key is to weaken and exhaust the enemy before charging home though. Thats why Riders are popular because you can achieve both functions with the same unit.

And this is true of any cavalry at any time. If the enemy isnt at least very tired, wait until they are before charging with a single unit. Obviously running off skirmishers is a different story.

Personally Im very fond of Dahae Riders.

But the point, to my mind of heavy cavalry is to force the enemy to break quicker. You can engage down the whole length of the line with infantry and no doubt eventually win by slugging it out and wandering light cav along behind the enemy. But using several units of heavy cavalry you can claim a massive and powerful numerical advantage in one part of the field that enables you to win quickly and with low casualties, then turn the victorious part of your army on the remainder of the enemy and wrap up a nice clean win.

Whenever Ive used the Romans in a historically accurate way they always take much higher casualties than their eastern rivals, including any Greek faction.