I agree with Craterus, I have tried many times to use Javelin Cavalry versus Heavy cavalry, even if hit from behind, they do practically no kills. Yes, they hurt morale, but your forces will be weaker overall from fielding such ineffective units, and besides, any competent player will have considerable amounts of infantry to support their cavalry, which will just make it so much harder to maneuver to use jav cav, and is far more effective in killing your cavalry, aswell as actually having a major use in the remainder of the game.

The only times I have found jav cav to be effective is in chasing down archers as they are quick and versus less experienced players who insist on charging their cavalry at them. I would not normally field them in games versus competent players.

Again, this is my opinion based on my experience, and I fully accept that I may be wrong, or not be using jav cav sufficiently well, but I have not been able to find a use for them that would justify their place in my armies over infantry. If you have any replays that demonstrate their effective use, I would be grateful if you posted them.