It's all down to preferences in many cases, although the number of redundant units are plenty (lithuanian cav for example). And sometimes it's down to the faction rooster.

Crossbows are certainly redundant, although I've been using them for a decade when high comes before teaching up to arbs. before I modded them to early and increased their rof slightly.

Mounted crossbowmen does it job decently on harassing and are good to deploy behind enemy lines, when the melee begins. But they are a horse archer substitute for factions not getting horse archers normally.

jinettes are rally good actually. They're among the strongest fast cavalry and got javelins to boot. Harass that heavy cav and draw them away or shower them with javelins. Then go and hit some flank somewhere.

The one thing to remember that foot javelin units are early units and meant as such. Thus pin that royal cav with spearmen and then hit him with javelins. I'm not sure why VI added such a number of them, since they don't have that much use (even with a good position, they run out of javelins quite quickly and aren't usually good at melee), but there's still places where they're useful. But they are mainly made to soften up tougher units in early.

Spears are better than polearms in these areas.
Early access: You'll get Chivalric Sergeants and halbs at about the same time. That's quite a while before polearms show up.
Arrow/bolt resistance: Silver CS vs the horde are really useful. I don't think I've ever seen a CS unit crumble and rout due to frontal arrow fire. Sure they get depleted after a while, but they're the stronged unit vs arrow fire in the game, thanks to their armour, shields and added unit numbers. The heaviest halb can handle this aswell, but they have the next problem
Speed: Mainly vs regular halbs and CFK. Those units are annoyingly slow. Can be countered, but extra speed is always useful.
Access: The faster halbs are all hard to get. Either faction or region speciffic.
Staying power: Spears makes an exellent frontline due to this. They last for a long time. And with their high charge values, they can do pretty good charges.

They're weaker in MP as far as I know, since many of thier advantages get nullified.

The boon with halbs are the ap (that get's better the longer the game goes due to increased armour) and that they got devastating charges vs cav, finishing the cav off much faster.

Normal pikemen are quite useless though. They lack many of the advantages normal spear gets and only gains it in bridge battles without arrow fire.