There are two major issues with spears in M&B:
1) AI fighters really like to cuddle with their opponents, I think spearmen keep some distance, but swords men will run into you as fast as possible and that makes your spear useless and leads to
2) In reality you can keep someone at distance with a spear. In M&B spears don't always have pointy ends, that means only if you're attacking with a spear it is dangerous. This applies to all pointy weapons in M&B, but you can at least slash your sword if you're too close to stab.
Balancing spearmen is a difficult task. I think the fairest way would be to allow a spearwielding fighter to keep one opponent away and increasing the spears' speed.
By the way, I intend to make hurling spears sideways possible in my mod. Don't know if it'll work because it'd only work on the right side, but giving spearmen an attack like that so they can hit them when their enemy is rather close should help them, too.
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Tuckles beat me to it
Ack, Berserker Pride and Tuckles were faster, so I'm going to reply a bit to their posts too.
@Berserker Pride
You're right about that and I'd have said the same thing too but from what I have gathered his mod is set in Southern Asia, with 'difficult terrain'. I doubt stopping horses with spears is a priority.
I only read it on wikipedia, but that doesn't necessarily mean it's wrong: Couched lances wouldn't have been totally impossible without stirrups.
The harvest arrows idea isn't bad, but the quivers shouldn't be 100% refilled. And it'd only make much sense if there were hostile archers having shot, too, because archers tend to shoot further than 20-30 paces.
@Tuckles, swords are more or less a kind of knife, which was, in fact, the first tool used by humans. And spears were only used for hunting before and have thus been weapons all the time. That's how I see it.
I guess noblemen liked swords because they were a symbol of, well, nobility and made them stand out from the poorer folk who couldn't afford them. Swords are also rather elegant, so that could've been a criterium, too. And of course, the choice of weapons probably was passed on.