I've read that the distribution of left handedness in the population is approximately 10% of men and 5% of women. There's definitely a premium to being left-handed as it helps for setters and right-sides, which are 1/3 of the positions on the court.

There is no downside to being a left-handed setter. I remember Caity Noonan at Cal, and her full left handed swing was way more effective than any left-handed dump by a right handed setter. At a clinic I went to, one coach noted that his dream came true when we ran a 6-2 with two left-handed setters. It wasn't so much that they could dump, but that they were both left-handed right-sides. I've even heard of mirrored setups done to give right-handed setters the opportunity to dump with their right arm swings.

Hana Cutura is naturally ambidextrous. She'll occasionally hit with her left hand, but she's said that she doesn't hit quite as hard. It would also mess up the setter if she would be set for both arms.