In addition to keeping the game balanced, the idea of a spearman defeating a tank one in a while isn't really all that unrealistic. Remember, the game's cartoons don't just represent one tank against one spearman. It's a bunch of tanks against a bunch of spearmen, who, on rare occasions, might be able to cleverly dig and disguise enough anti-tank pits, leap out of enough trees at overconfident tank crews who have left that hatch open, steal enough guns and hand grenades off of the enemy dead and figure out how to use them effectively, etc., etc., to pull off a victory in a battle. Not a war, necessarily, but a battle.
There are plenty of times in history where a technologically much inferior army, usually with better morale, motivation, and knowledge of the local terrain, was able to make life pretty difficult for their opponents. Think Ethiopia against the Italians or Majahadin against the Soviets. Neither was a case of people literally thrusting a spear at a tank, clank clank clank <boom> clank clank clank, as in the Civ animations. But spearman v. tank is a reasonable metaphor for that sort of thing, in the same way that building city improvements one at a time is a reasonable metaphor for setting the educational, religious, military, and so on priorities for your society.
<heresy>If anything, I think that tank is unrealistically ~overpowered~ against low-level units.</heresy>
Cheerfully submitted,
Michael4000