However, then you have to buy back in and time it.
If I understand those beancounters properly, the idea is that as you age and need to use rather than grow the investments, you limit your risk by shifting more and more of your portfolio to low risk, cash like instruments. You basically only keep as much as you can afford to lose in the market, that way you're shielded from crashes and recessions..
Obviously, this presupposes that you've reached your financial goals by the time you reach retirement and can "afford" not to take risks that could jeapordise your lifestyle..