I'm on an instance that has downvoting disabled. I can't downvote. But, I also only see upvotes, and posts even on other instances are sorted by raw upvotes (not upvotes-downvotes). A downvote does not affect my experience in any way.

That sounds like a bad thing.

I disagree. Downvotes rarely add anything of value to the discussion that can't be expressed simply by not upvoting. :)

I respectfully disagree. Downvotes add a way of gauging the percent of people who support/don't support a comment. Let's say I'm asking for advice about which product to buy. With an upvotes-only system the upvote count is biased towards the earliest comment, whereas with an up/down vote system, the ratio helps you detect comments with heavy bias or blatantly wrong facts. So an upvote/downvote system makes it easier to tell the credibility of a comment, basically allowing you to indirectly gauge the opinion of the community rather than the one person who commented.