If spears were so useless in one-on-one fighting, there wouldn't be two major polearm-wielding gladiators (retarius, hoplomachus) in the standard line-up against all those swordsmen (samnite, murmillo, thracian, etc). Because a one-sided fight is boring, and once they got past the notion of gladiatorial matches being funeral games and into public entertainment, boring is not what people want to see.