In good complex games, class balance is not important because there are many objectives in the game. 1v1 PvP is not a good measure of class balance. Even if an Archer can kill a Swordsman at far distance, it does not make the Archer better because a Swordsman can kill the Archer at short range. The Archers are good at long-range fighting while the Swordsman is good at short range fighting. A better example is an Archer vs. a Medic. The Archer will always win, but people will still want to play the Medic because Medics are needed in teams.
It is only in simple games, where the objective is only to kill the other players, that class balance becomes very important. If Zerg always beats Terran early-game, then it is pointless to play ZvT games. There are many degrees of freedom that determines the strength of a race: cost per unit, unit speed, size of unit, attack strength, hit points, special abilities... When these are pitted against one another on the simple [new HP] = [current HP] - [attacker's attack] algorithm, not one class should have a solid advantage.
It is only in large 3v3 or 4v4 Starcraft games that other objectives materialize. If Terran always loses to Zerg early game, then the Terran's Zerg allies can protect the Terran in early game. The Zerg can therefore play the objective of early defender, as opposed to just killer of opponents. In 1v1 games, this is not possible to do.
Also, it is bad when players can only play with one class. Chess has this: black pieces move the same as the corresponding white pieces. An alternative would be where all the black pieces move like pawns and all the white pieces move like bishops. The patient player who wishes to squeeze the opponent to death would want to play the black pieces while the aggressive player would want to play with the bishops. This customization is good because the game would be more personalized for the players. Chess does offer this in the form of aggressive and passive opening positions, but not in the form of pieces. The pieces are orderly enough in the meager 64 squares and one-move step-by-step play that positions can be highly customized, while the units in Starcraft can only pull off positions such as flanks, surprise attacks, etc.
So in summary, in good complex games like World of Warcraft, class balance is not important because there are many objectives such as healing teammates and making weapons that can make the "weaker" classes fun to play. Class balance in games like Chess and Starcraft is important because there is not enough degrees of freedom to offer objectives other than kill Enemies.