In the long run, there are quite a few mercenaries which outclass Spearmen, even in the middle levels, so I would not recommend using them once these other mercenaries become available. And while the class is useful for formation builds which make use of a large number of Gunners and Spears, this may be the Spearman's only value, in the long run.
I find the utility of Spearmen in any party layout other than Spears/Guns to be lacking rather substantially; in any party which I've face which has one or two Spearmen in them and no gunners, it's generally a simple matter to dispatch the Spearmen, and while their damage output is definitely a hassle, it is not as serious a matter as the damage of a Gunner would be, and generally your front-rank mercenaries are able to take the brunt of their attacks without falling. Spearman make for a mushy front line if you are using them as front-rank tanks, and ultimately their lower survivability results in their generally quick death, mitigating any damage that they would have dealt in the later rounds of a fight.
Spearmen are good for Spear/Gunner builds in the early game, but I really don't see anything positive about the class beyond those builds. I really consider other classes to be better choices for the early game, such as the Swordsman, Viking, Monk, Shaman, Artilleryman, and Gunner. It's more likely that you'll keep a Swordsman, Monk, Shaman, Artilleryman, or Gunner into the middle levels and late game and be happy with their performance, as while Spearmen are decent, they do not shine as brightly as some of the other more useful classes.