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
Likes (and dislikes) are needed mostly for viewers who might want to feel helpful.
Dislike mobs do more harm. They probably ignore dislikes already.
1. Videos which promise to show something that never actually appears
2. Very short videos which are difficult to not watch in their entirety