Livre d'Or

I don't quite see why posting a lengthy comment to someone's post would be considered to be taking the discussion off-topic. I mean, assuming that the comment is about the post itself. Surely the point of making posts about contentious subjects is to instigate discussion in the comments? I wouldn't be offended as such if someone did this to me, but I'd be a bit non-plussed and find it quite inconvenient to have to continue the discussion in their journal when it started in mine.