Left-handedness does indeed confer benefits, as any fencer can tell you. Lefty gladiators in the Roman Arenas were universally feared, though that was, at least in part, due to the tactics of the time.
But the advantage disappears when lefties are as common as righties, since it depends on the habits of the opponent. Which may explain why lefties are a constant minority, since, if they got too common, their tricks were less useful. As their numbers diminished, they became more successful, and more numerous, until that upper limit was reached. Eventually, some balance would be achieved.
I'm not sure that lefties have such compensations today, what with corkscrews and gravy-boats replacing the sword and buckler. Perhaps we'll see them disappear. They might die sooner, anyway. And after all, "sinistrality is thus nothing more than an expression of infantile negativism and falls into the same category as contrariness in feeding and elimination, retardation in speech, and general perverseness in so far as the infant with meager outlets can express it", as Abram Blau would have it.
