You are right. The pikemen should have been supported by adequate number of light infantry and heavy cavalry. It seems that after Alexander and Pyrrhus, later Hellenic armies ignored it. In my humble opinion there was the lack of effective heavy cavalry in the late Hellenic armies. At first, the heavy cavalry was replaced by war elephants, and it was a good idea. But when elephants were abandoned the Hellenic pikemen were weakened greatly.The pikes not in the first rows provided some protection from missiles and like other troops, they had their own support troops to cover them against light troops. Ideally, one would want their own light troops to pick up the slack as far as missile protection goes rather than their heavy infantry being forced to sit under fire.