The reason for this is simple, and two-fold: the spear is cheap and easy to make, and it is extremely effective. I've said it before, and I'll say it again: it's not for nothing that the spear is the King of Weapons.
I'm not sure this is really necessary, but...
The most basic spear is simply a sharpened stick, 5-8 feet long. At it's most advanced, the spear has a steel bladed head, 18" long, with lugs at the bottom of the head, a sharpened steel buttcap on the base, and a polished and lathed ash shaft 6-7 feet long.
The butt cap is an important point - many people don't realize that medieval spears have a sharpened steel spike covering the bottom. This makes the spear significantly more dangerous than if it only had the bladed head at the top.
|A spearhead and buttcap.|
The lugs allow some more parrying options, but they're certainly not necessary. Fiore's spear treatise doesn't show them except in the bit on a spearman vs. a horseman.
|Spearhead with lugs.|
Offence with the spear is really fairly simple - insert the point into the face of your opponent. Keep in mind, though, that the spear has a point on both ends. Since any defence against the spear will involve knocking the spearhead offline, a common tactic is to absorb and use the energy from that knock to spin the spear 180° and attack with the buttspike instead of the head.
Much of the stuff that applies to the halfsword (discussed in my earlier post on the longsword) also applies to the spear, as halfswording is really just turning your longsword into a 4-foot spear.
There is currently debate about whether the spearhead or spear shaft were used for striking as well as thrusting. There is little direct evidence for spears being used for striking, but arguments can be made for it. Viking sagas speak of "hewing spears" which were used for cutting. What these are is unknown - it is possibly just a normal spear, a spear with a long head, or something more like a glaive or naginata (i.e. a sword on a stick, as opposed to a spear, which is more like a dagger on a stick).
From handling spears, I can safely say that being bashed with the end would be pretty awful. I have little doubt that a solid hit from a spear could break bones in an unarmoured target.
There's also the fact that in Fiore's medieval fighting system there's a certain universality to the techniques - since the sword can be used like a spear, it's not unreasonable to see the spear sharing some techniques with the sword.
Wiser people than me disagree the the spear is used for bashing, though, so take the notion of bashing with the spear with a grain of salt.
Striking sideways with the spear definitely comes into play on the defensive. Most of Fiore's spear guards have the point off the line, pointing up, back, or to the side. The point-forward guards, while having the point forward, can still generate significant sideways force with a passing step.
The basic defence with the spear is to turn and swing your spear to strike the opponents weapon aside, ending up in a position where your point is directed at the opponents face or neck and your spear is between you and the opponent's weapon. If you over-parry (i.e. your spear keeps swinging past their face), you can keep that energy going and turn the spear right around and strike their face with the butt spike.
|The master has successfully parried from the previous position. The momentum of the opponent's attack has carried him right onto the master's spear.|