Earlier quoted context omitted.
They're helpful when searching for tutorials especially on topics you don't know much about. It's a way to navigate the big mess of unknown to me channels. For example, if I'm looking for a video on how to fix something at home, the downvote ratio can quickly tell me if someone is giving bad advice or just rambling and wasting my time. For channels I watch regularly downvotes aren't relevant at all.
How often do you check the supposed "bad tutorials" to verify that this assumption is actually true?
Not often, the whole point of this heuristic is to watch as little as possible and still get the information I need. I don't really care about false-negatives unless a very-downvoted video is the only useful video on a topic (which is unlikely). It's just a useful shortcut, not a way to perfectly classify videos.