But if someone's got a bead on a 99.99% efficient market, they can make a lot of money, whereas if you say "Ah ha! The market is not perfectly efficient! Here, I can make money doing this!", you probably won't make much.
(Then there are those who think the market is like maybe 10% efficient, in which case my response is, if it's so inefficient, go make money. That would imply all sorts of arbitrage opportunity. Why slave away in whatever job you may have now when there's such an inefficient market just waiting for someone smart & clever to pluck it? The answer is that where ever the market may be on the range of 0-100% efficient, to the extent that such a measurement is even particularly definable, while it may not be exactly 100.00000%, it's much closer to that than the 0% side.)
(Remember, efficient doesn't mean good or moral, which I think is the most common mistake I see on HN. It means something much more like "there are no easy arbitrage possibilities". It hasn't got anything to do with "goodness" or "morality" any more than analyzing gravity or electromagnetism in terms of its goodness or morality makes any sense.)