EFF/KRF: This strategy does reduce your downside exposure to equity, but the insurance is not free. Buying the put leaves you with less to invest in equities. In other words, you have to sacrifice part of your upside return to protect against downside loss.
You can get exactly the same payoff by combining call options and Treasury bills. But the put-call parity relation says that the cost of this portfolio matches the cost of the stock plus a put. In either case, the insurance is not free.
It is important to remember that someone has to bear the downside risk on your equity portfolio. The risk does not go away when you insure your portfolio by buying a put, it is simply transferred to whoever sold you the put. You should buy protection only if you think the insurance is underpriced or you have an unusual utility function. If you are motivated by simple risk aversion, you would probably be better off reducing your overall allocation to equity.
Eugene Fama and Ken French are members of the Board of Directors of the general partner of, and provide consulting services to Dimensional Fund Advisors LP.