Downvotes have some advantages over blocking. They:

• Indicate trends of spam (as @morphballganon@mtgzone.com pointed out).
• Allows minimizing exposure to unwanted content from a given account without blocking out exposure to all that account's content. (Accounts may post a mix of content I do and don't like. Blocking throws out the baby with the bathwater.)
• Help disambiguate the popularity of content vs. reach of content. (Relatively few upvotes compared to other posts could be due to community obscurity, or posting at a poor time of day. It doesn't indicate whether the community likes the post, or how you should adjust your posting habits to increase appeal. Counting downvotes helps you understand how many total people viewed a post, and the percentage of those who appreciated it.)
• Are lower friction. (They take fewer clicks.)

Well that's heartwarming.