If you have not edited it enough for the reader to stay focused on the narrative instead of the copyediting mistakes. Or if you do not think you have a draft that is complete enough for the critique to be of any use to you. Early critique can derail a writing process, but not getting any critique means the writing may lack relevancy to the audience. No easy or good answer since it all depends on the author, the editor, and the audience. For a blog post, the moderator is good enough, for anything else find a good editor.