It is generally accepted that efficiency represents the optimal, aspirational state for any market. Efficient markets, which feature many buyers and sellers and perfect information flowing between them, determine the “right price” and hence allocate society’s resources optimally.
Those are indeed positive features. But every good thing is like a face caressed by the sun. The rays that light and warm the face automatically cast a dark shadow behind it. The shadow of an efficient market is increased price volatility — quite the opposite of what we expect from efficient markets.