Not seeing the original comment to which you're referring, I can't offer any opinion on it in particular. However, I generally disagree with the statement "if a comment gets down voted but starts a wave of discussion surely it has value." Trolling is, at its heart, the creation of discussion by posts of negative value, usually by eliciting a strong emotional response from readers. Most troll-posts don't have value to those trolled (though there may be exceptions).