As a YTM hater, here are my reasons:

1. They closed down Google Play Music, which was an objectively better service, and didn’t have feature parity in YTM when they did so. They had promised it, so regardless of arguments towards what it might have now or what features might not be needed because XYZ reasons, they promised that all the features from GPM would be in YTM before GPM was sunset and that was simply not true.
2. Maybe this has changed, but at launch, I couldn’t “Like” a song to influence my generated playlists without it “Liking” the video on main youtube, which influences my video recommendations. I don’t want YouTube serving me exclusively music videos because it sees my YTM Likes and vice versa.