Does Friending someone requires the person to accept? If so, I guess it could be harmless. Especially if a User setting is added that lets you control "Who can see my ratings": [Friends/Nobody] with one of the two being at default. There could even be an [Everybody] option.
And on top of that, we could even have a way to selectively disable the possibility for someone to see our ratings. Like in IM apps to selectively disable the possibility of seeing "Last seen" times per person. It could also be disabled automatically for Blocked users.
I think sharing opinions with other people, in a voluntary and safe way, can only be beneficial.