It's rare that I use the pantodapoi for field battles but they have their uses. To absorb missiles, to fight skirmishers since they are quick enough to catch them. They may have skirmisher stats but their spear has better lethality so they slaughter them. Lets not forget that it takes a pretty good cavalry unit to beat them in a straight fight. They slaughter most light and medium cavalry and cost a fraction of the cost. Even good cavalry takes a while to rip through such a large unit size. I use them to bog down enemy forces or to fight on the flanks, either by flanking or turning the tide of a cavalry duel.

Personally I find the costs of units a bit messed up. Get a better type of a unit and it's increased cost is by far worth it for the quality boost. Once you get Hellenic heavy spear men the regular version just isn't worth the cash. Once you have a better alternative to the pantodapoi there is no reason to have them. You'll only use them if your the Seleucids because you can't make much else in most your empire and need the garrison size. Still, I'll take these guys onto the field if I have nothing better, as long as you don't stick them on the front line they will be more than worth their cost. (which isn't much so you can replace them)

Hellenic archers get my vote for the worst unit. Only use they have is running around the flank and shooting the enemy in the back, which is just cheap so I don't do that... Still I use them, just because it doesn't feel right not to.

I rate javelin cavalry pretty low. All they are to me is a way to get some cavalry on the field if your too poor or under developed to get anything better. But when it comes down to it a shaken or wavering enemy will break if anything hits them in the back and not much is fast enough to flee from them once broken. I like to make them hunt fleeing generals so I don't have to deal with a full body guard next turn.

Hmm, seems I'm just rambling now...