As an epigrammatic means of defining appropriate civility in exchanges on Quora: it is a very good ideal, and one that is lacking in many online fora. My libertarian heart has chafed against it, but I have appreciated how it has helped me off the ledge in interactions with other users on more than one occasion.
There are areas for interpretation which cloud it, of course. A forum that uses the “Be Nice” to ban sarcasm or swearing is not a forum I have much time for. The way it is used to suppress discussion of moderation actions is one I have consistently resented as specious. And any extension of BNBR to public figures, I regard as illegitimate.
But as an ideal for one-on-one interaction, yes, it is a good thing.
Its implementation, on the other hand, is as flawed as you’d expect. No transparency, rulings that leave you scratching your head, inconsistencies, vulnerability to vendettas.