The modern equivalent of the question would be: "The rifle is a vastly superior weapon to the pistol. Is this true? If so, what purpose does the pistol serve?". The analogy isn't exact, but still useful.

The first part: Is the spear superior? What did writers who compared weapons say?

From England, George Silver wrote:

The long staff, morris pike, or javelin, or such like weapons above the perfect length, have advantage against all manner of weapons, the short staff, the Welch hook, partisan, or glaive, or such like weapons of vantage excepted

and he notes that such long spears (morris pike and javelin) have difficulty against two swordsmen, since one will get in close because the long weapon is sluggish to turn. However, shorter spears (about 8 feet long)

The short staff or half pike ... have the advantage against the battle axe, the halberd, the black bill, the two handed sword, the sword and target, and are too hard for two swords and daggers

are fast enough to defeat two swordsmen.

From Japan, Miyamoto Musashi wrote that the most effective hand-to-hand weapons on the battlefield were the spear and naginata ("halberd", but "glaive" might be a better translation):

The halberd is inferior to the spear on the battlefield. With the spear you can take the initiative; the halberd is defensive. In the hands of one of two men of equal ability, the spear gives a little extra strength.

Whether on the battlefield, or one-on-one in an open space, the reach of the spear provides a large advantage.

Does the spear have disadvantages? Yes! It takes space to use effectively.

Musashi:

Spear and halberd both have their uses, but neither is very beneficial in confined spaces. They cannot be used for taking a prisoner. They are essentially weapons for the field.

In addition, a spear is inconvenient to carry. It will occupy one hand (except cavalry will sometimes sling a spear over a shoulder using a strap - useful for mounted archers/gunners), and is difficult to bring indoors. It is a weapon for the outdoors - for hunting or for the battlefield. It isn't a weapon you can take everywhere with you.

A sword (of typical size), on the other hand, can be worn everywhere. Wear it on your hip, and you have both hands free. Not so heavy as to be an excessive burden, and quickly brought into action. Further, unlike a spear, it can be used when space is limited. It is the weapon that both Silver and Musashi focus their instruction on. Both writers noted the advantages of the sword:

Silver:

The short sword, and sword and dagger, are perfect good weapons, and especially in service of the prince. What a brave weapon is a short sharp light sword, to carry, to draw, to be nimble withal, to strike, to cut, to thrust both strong and quick.

Musashi:

The long sword can be used effectively in all situations.

The convenience of carry has made the sword a useful everyday-carry weapon for civilians, and a widely-used sidearm on the battlefield. On the battlefield, it has served as a sidearm or secondary weapon for infantry whose main weapon was a spear or other polearm, or a ranged weapon (bow, javelin, musket, etc.), and as a sidearm or secondary weapon for cavalry whose main weapon was a lance (in which case, a second weapon is necessary, since lances break or get stuck in targets), pistol, bow, etc.

Swords have seen use as primary weapons. Rodeleros ("shield men") were used by Spain in the 16th century, equipped with sword and shield. Cavalry, while often equipped with pistol, would often fight hand-to-hand with sword after firing their pair of pistols. Roman soldiers would fight with sword and shield after throwing javelins. The Qing retained sword-and-shield equipped soldiers (e.g., tigermen, and sword and shield equipped militia) in their predominantly pike/spear and musket armies.

There was also use of large two-handed swords as primary weapons on the battlefield, in Europe, China, and Japan. These don't have the everyday-carry convenience of regular-sized swords, and the wielders of such swords would typically carry a conventional sword as a sidearm.

Further to their use as fighting weapons and self-defence sidearms, swords were also used as a display of power and wealth, as symbols of rank.

To return to the rifle/pistol analogy, note that spear is less convenient to carry than a rifle. On the pre-modern battlefield, the reach of hand-to-hand weapons mattered more than on the modern battlefield (where firearms are the standard weapons). This meant that a sidearm of respectable reach was more important. (For the modern soldier, the rifle-as-club often suffices as a sidearm.) When swords were relatively rare, large fighting knives were often carried as sidearms (or axes, etc.). When swords became more common, swords became the usual sidearm. The pistol sees military use today mostly for soldiers who do not usually carry a rifle (artillery, service personnel, military police, etc.), and is not paired as a sidearm with a rifle in the way that sword often accompanied spear. However, for non-military use (e.g., civilian self-defence weapon, police weapon), the pistol performs many of the fighting/defence functions the sword did as a non-military weapon.

References:

Miyamoto Musashi, Gon Rin No Sho (A Book of Five Rings), 1645. Quotes from the Victor Harris translation (Allison and Busby, 1974)

George Silver, Paradoxes of Defense, 1599. Online at http://www.pbm.com/~lindahl/paradoxes.html