Dislikes are good not just for identifying bad videos, but also good videos. You can see it in the like/dislike ratio. The higher the better, videos with ratio around >98-99% have very high probability of being good etc
Comparison of likes to views works well enough. Actually, they don't even need likes, they can understand good and bad videos just by analysing how long viewers spent viewing the video.
Likes (and dislikes) are needed mostly for viewers who might want to feel helpful.
Likes (and dislikes) are needed mostly for viewers who might want to feel helpful.