In both cases you have the same data loss problem. Without downvotes you can't separate uninteresting from harmful and with downvotes you can't separate harmful from low quality which is why voting etiquette is recommended:

> If someone spends the time to make an honest effort to answer a question, but it's not that great an answer, just don't upvote them

That means voting etiquette is part of the vote mechanic itself; a critical piece that makes the technology work rests in the minds of its users. If you've spent any time on Reddit, it's obvious this is a common source for trouble as a person seeing an up/down binary will naturally think this is just a personal value assessment. This is amplified by the fact downvoting has actual effects, lowering the visibility of a contribution encourages downvotes to hide content the user thought was poor quality.

This is why FB reactions happened. A mechanism intended to reduce noise has to guard against data loss to some degree or the mechanism itself produces noise, as it is with lossy audio formats.