It's been discussed before in shady corners of other forums and these, but typical medieval armies used a lot of spears for their general infantry. Now obviously the 7 kindoms have access to much better equipment than the usual historical army and it has high emphasis on knights, but still...as effective as swords are, spears make a lot of sense in a general infantry unit, even in a fantasy world.

Maybe they are keeping them for some faction? Do the books shed any light on this? The Unsullied are certainly going to fight in phalanx formation.