It discourages people from voting if they’re concerned about other people seeing their activity. This could result in a lower quality of scoring for posts.
I strongly disagree with that. I think showing downvotes makes your votes more relevant. If something has 10k up votes and 10k down votes, it’s probably a decent post. If it just shows 10k up votes, or 0 net total, the score doesn’t reflect the nature of the post.
At the individual level, it lets you know if someone is just trolling. That’s also a plus as far as reputation goes (not sure how people are scored here, or if they are).