In the longer term, Spearmen ultimately end up being one of the less viable classes to keep in your formation. The Swordsman is a better tank, and several even better tanks arrive in the late game, such as the Exorcist, Beast Tamer, Spartan, and Lady Knight. The Spearman's main strength, their damage output ultimately pales in comparison to later mercenaries who can dish out more damage than the Spearman, such as the Cannoneer, Witch, Beast Tamer, and Inventor, along with a number of other more damaging classes.
Spearmen are useful in the early game for gunner/spear builds, but by level 60 or so they become obsolete as a large number of other more powerful mercenaries enter the scene. There really is no final build in which Spearmen are a good addition, in the final game they can only be a liability, and I personally feel that it's a better choice to raise up your Monk, and any Gunners or Artillerymen from the very beginning, if you plan to use them in your final build, otherwise you're just going to end up throwing all those Gunners and Spears away in the late game when the Lady Knight, Spartan, Witch, Exorcist, Beast Tamer, and all of the other mercenaries become common at your level.
Parties with all of these mercenaries in them ultimately have a ton more flexibility than a build like the Spear/Gunner formation, and things like the Princess's ranged damage debuff or stun formations could easily wreak havok upon such a formation, leaving the Spear/Gunners with no counter to these superior Class C builds.
