Prodromoi are viable flanking cavalry. Their spears, shields, and linen armor give them some staying power, but they should not be used to in protracted melee for any great amount of time.
Prodromoi are the lightest of the Diadochoi melee cavalry and can be counted upon as superb flanking, raiding, and screening cavalry. They are well trained for light cavalry, but are still lightly armed and armored. They are not meant to be used as shock cavalry and will not roll up an enemy battle line in a thunderous charge, but instead make viable flanking cavalry which can quickly provide support where it is needed and be used to counter light missile troops of both the foot and mounted varieties. Their spears, shields, and linen armor give them some staying power, but they should not be used to in protracted melee for any great amount of time.
Historically, the Seleukeis and Makedones used the smaller and weaker members of the nobility to form a quick and powerful light cavalry force that could decide an engagement on its own. Since these men are nobles, they have good staying power, as was evidenced by their widespread use in many battles. They are quick, and can be used to chase down missile cavalry when there is a need, due to the fact that they ride fast Thessalian horses and are relatively slight statured men. The Seleukeis use them as an effective counter to the Parthoi horse archers, much like the Hungarians of a later era did against the Turks.
One of the best lancer cavalry. The role of these cavalry men is to provide a supporting cavalry charge against the enemy, they are also able to battle light cavalry and light missile troops due to their good armor, fast horses and good morale.
Their speed also makes them effective at chase routers, especially for killing a fleeing general. Two of these cavalry men can "sandwich" charge an enemy cavalry, ensuring what's left of the enemy won't make it back alive, effective for ensuring an enemy general's death. These men, when partnered with heavy cavalry such as the Hetairoi (Companion Cavalry) or Hellenikoi Kataphraktoi (Hellenic Cataphracts) makes for a deadly anti-cavalry unit or a decisive cavalry charge against weakening enemy infantry.
However, they do not have the power to last in prolonged melee and should be used primarily as lancers.