When I made the case as to why you need to have some of your portfolio allocated to bonds, I outlined the conditions necessary for a 100% stock portfolio to make sense.
Market timing is a nice story. However, research suggests that the stock market is efficient. If the stock market is efficient, all available information will be reflected in stock prices.
If you don’t have a multi-decade time horizon to retirement or you aren’t comfortable with massive short term volatility than you absolutely need to allocate a chunk of your portfolio into bonds.
The standard stock-bond allocation for a diversified portfolio has been a 60-40 split between stocks and bonds. Historically, a 60-40 bond allocation has done quite well.
Right now I still have 100% of my retirement portfolio in stocks. As I get older I will become much more conservative and begin allocating more of my portfolio to bonds.