I have actually thrown a javelin and it was perfectly balances for throwing, however you couldn't jab with it in the throwing position, overhand. The enemy would have to come a lot closer to you with a spear overhand because you come down with the spear. When you use it under hand you get the full extent of the pole, hence the reason for all the different pole weapons. Also the Hoplite fought in the Phalanx formation, front line had spears next to them, not over the shoulder, and the hoplite next to them would protect them with their spear, they couldn't do that if it was overhand. But I guess everybody already knew that.