Agree: timing the market is hard, but it's not impossible, particularly A) given that many big (institutional) investors are not at liberty to enter and exit (almost all mutual/index funds have prescribed what proportion must be in equities, say.) B) given that not all markets are as efficient as the EMH makes them out to be. For example, China fell precipitously from June 2015 ( http://www.bloomberg.com/quote/SHCOMP:IND ), and The Economist (AsiaPac edition) had a cover on 2015-05-30 proclaiming "China's overvalued stock market" ( http://www.economist.com/printedition/2015-05-30 )

To conclude, I think it is prudent to review your asset allocation maybe once or twice a year, and shift things around a bit.