Flags should be used for abuse/plagiarism/disagreement on rewards, when a flag is used it takes away monetary value from a post or comment.

Downvotes should be used on "disliking a post" to show that you don't agree with it/don't like it/whatever, but it doesn't take away any value from the post itself.

Just because I disagree with someone doesn't mean I want to take value away from their content.