I keep them private because I don't want the kind of content I consume to intersect in any way with the content I produce. Same reason I keep 'Liked Videos' and stuff like that private. I use my Youtube account for consuming content as well as creating it, and I don't want anyone watching my content to then go and be like "why do you watch all this random garbage" or worse "hey I saw you subbed to this person, my content is similar, sub to me!"
I feel like it takes away from what I'm trying to present/convey with my channel.
edit: Then again, my actual sub count is private, too. Maybe I'm just a really private person. I don't like there to be information about me I can't control directly.