The simple truth is that light, javelin armed cavalry are not battle winning units. Used correctly, however, they are good auxiliaries (particularly the Thrakian light cavalry). I find them a nice compliment to heavy cavalry. As Makedon, I often deploy my Companions, Thessalians, and prodomai with a light cavalry unit on their flank that can circle around the enemy heavy cavalry after mine deliver there devastating charge and fire missiles into their rear. Then, while the heavy cavalry turn in and roll up the enemy flank, the light cavalry can pursue the enemy cavalry or skirmishers. This system, combined with my solid phalanx line which can hold against anything in time for my cavalry to arrive, has worked flawlessly in my current Makedonian campaign.