Daniel Dennett is one of the great free-thinkers of our time, often mentioned in the same breath as individuals like Richard Dawkins, Sam Harris, and the late-great Christopher Hitchens. Unlike Harris and Dawkins, and more like Hitch, Dennett is not a scientist and thus talks more about philosophy than science (although when he does talk science it's interesting and he largely gets it right). He mentions a lot of fascinating ideas about the human mind in his writings and speeches, but like his aforementioned colleagues he often renders sharp blows against religion (as well as other rubbish philosophies). During one of his speeches (I cannot recall which one, it's on YouTube) he said the phrase that is the title of this write-up. He is also quoted as saying it in a book by Nicholas Fearn - The Latest Answers to the Oldest Questions - A Philosophical Adventure with the World's Greatest Thinkers, although in a slightly different context than the speech.
Whilst Mr. Dennett was talking about religious studies and people who get degrees and PhDs in the subject, he mused the worthiness of that pursuit. Here I must paraphrase: "You all know the phrase 'if it's worth doing, it's worth doing well?' Well, here the corollary phrase applies: 'if it's not worth doing, it's not worth doing well.'" That was met with appropriate audience laughter. And from myself. It's so true, isn't it? Especially if you're talking about people who put a lot of effort into apologetics and studying nonsense like Creationism and other aspects of extreme religiosity that are dripping with profound stupidity. Useless pursuits are not worth extra effort - or any at all.
Dennett calls this "Hebb's Rule," so-named after the Canadian psychologist Donald Hebb, whom Dennett cites as where he originally got the phrase.