Just watched as much of a video interview with Jack Bogle from Vanguard as I could stand. He is responding to claims from Mark Cuban that diversification and buy and hold don't work. Bogle of course built Vanguard on the idea of indexing, buy and hold, and diversification. He had the advantage of timing as the market went up in pretty much a straight line from 1982-99 and indexing is better than most of the actively managed funds out there. However, if he came up with this idea in 1999 he would be thought of pretty differently in the investing world because investors in his funds wouldn't have made much, if anything over the past 10 years or so.
Buy and hold works in an up market, along with anything else, but why give back all or your gains when the market goes down? Traditional diversification doesn't work either. Adding bonds to a portfolio can reduce risk but it sacrifices return. Other asset classes like international stocks and commodities tend to highly correlate to U.S. stocks in a downturn. It is time for investors to put these old ideas to bed and pursue strategies that seek to be in harmony with the trends of the market and strategies that are diversified by methodology and time frame.