I wonder whether superstitions don't have a (slightly) harmful psychological effect. For example, people could make irrational and suboptimal decisions because, say, their horoscope said not to do business today, the first taxi that came along had a "4" in the license plate or it's an unlucky day, so they're going to be withdrawn and moody, etc.
Would productivity improve, or reported happiness increase if superstition disappeared? Hmm. Would people have a more positive and fruitful attitude if they believed their destinies were in their control, instead of a god, a number or fate having that power?
Sure, live and let live, but let's not pretend there's anything positive about superstitions. Especially when they involve not cutting fingernails or hairs growing out of moles. Yuck.