I know there were some users downvoting SBI users back on Steem, but didn't realise they were still at it or even doing it here.
I echo a few of the answers already here. It's a fantastic help for newer users and retention. There is the scope for abuse, but Joseph is generally on top of that. There also seems to be a limit on upvotes for one post, so it's not like you're going to get over rewarding a mediocre post.
The jury's still out for me on whether UBI will work in the real world, though.