1. Their cost.
2. Their bonus against cavalry.
Yes, spearmen were units that cost only food and wood (and pitiful amounts of each) to produce. This is very true, historically, since peasant levies were usually comprised of spearmen. Agricultural implements are very easy to turn into polearms.
However, in Age of Empires 2, a single spearman is a danger to a single horseman, leading to the creation of small, dynamic spearman armies that interecept horsemen, as well as blending spearmen and swordsmen to provide a better chance against cavalry. This is where realism fades. Spears do *not* have any intrinsic bonus against horsemen, in and of themselves. In fact, a lone spearman is at a disadvantage against a mobile horseman, since a spear is very long and unweildly.
There, I said it. But if spears are so useless against horsemen, then why are they considered *the* anti-cavalry weapon? The answer here is many-fold:
All infantry suffer due to the mobility of horsemen. So the key to fighting horsemen is to remove their mobility. This is accomplished by large formations of rather immobile spearmen. With a wide enough formation, horsemen have trouble flanking you and decimating your formation.
Yet if the formation was key, then couldn't swordsmen do the same as spearmen? Yes and no. The Romans used tight swordsman formations with great success against disorganised germanic and gaulic horsemen. Yet these horsemen could wield long blades due ot their high position in the saddle. In order to reduce friendly casualties, foot soldiers could only wield small blades. So horsemen would often outrange infantry, especially if they were wielding spears (which were very popular).
What spearmen can do is called "hedging". The front row of spearmen extend their spears, and the tips end up about ten to twenty feet in front of them. The second row extends their spears, and the tips end up six to sixteen feet in front of the front row, and so on. Spears when used from a tight formation form an impenetrable barrier, stopping not only horsemen but infantry, too.
And on top of that, spearmen are cheap, so it is very very easy to mass the amount of spearmen that you would need to create an effective spear formation.
If Age of Empires III raises the population limit as has been the trend (if I may cite a trend in which only two games were involved), then spearmen must be changed to fit the new engine. Spearmen should be very effective defensive units, but their weakness should be missile weapons, not swordsmen. Spearmen effectively ended swordsmanship on the field of battle in the late middle ages, when massive spearman armies could not be breached by swordsmen or cavalry.
Please consider making spearmen weighted even more towards the defense. As they are right now, they make armies very ahistorical by being assault troops designed for pouncing on and shredding enemy cavalry. That is what camels are for. Spearmen would be much more useful if they could form an effective "flesh wall" to protect archers and musketeers.
Regards,
Dibujante