During accumulation, not much reason.
Right before and right after retirement you need to worry about sequence of return risk.
100 % stock portfolio: Average returns = 10%. Worst year performance: -43%.
What if that worst year is the year you retire, what if the following year is also bad, and what if it takes several years to recover. Multiple times in history the stock market has been down more than 50%, and on multiple occasions we've experienced negative returns over 10 years. If you started with a 4% withdrawal rate and the market tanks 50% you now have an 8% withdrawal rate, and unless the market turns around quick your balance will keep dropping. At that point the market is unlikely recover fast enough at that point to cover your withdrawals for 30+ years.
Or...
60% stock, 40% bond portfolio. Average returns = 8.8%. Worst year performance: -26%.
Historically you are giving up 1-1.5% return per year, but you significantly cut your worst year performance. Here is another way to look at it. If you retire using the 4% rule then you have 25X expenses at retirement. With a 60/40 portfolio that means you have 15X expenses in stocks and 10X expenses in bonds. If the stock market crashes the same year you retire then you could live off your bonds for 10 years, not counting dividends and interest income. That gives the market plenty of time to recover. You could also rebalance from bonds into stocks.
Side note: 100% stocks and a 60/40 are unlikely to average those same returns over the next 10 years, but that's another conversation.
I'm at the age where 'they' say I should have a decent % in bonds, but this is a terrible time to be in bonds.
The FED has made it clear that they are raising interest rates, bonds a very likely to lose value in
a rising rate environment.
From 2003 to 2006 the Fed funds rate went from 1.00% to 5.25%. Vanguard Total bond fund still managed to have positive returns every year. Why? Well because a well diversified bond portfolio doesn't fluctuate nearly as much with interest rates as people like to make it seem. Bond prices were down 2% over a year isn't a story worth panicking about, but the news needs to have something to panic about so they go on and on about the impending bond market crash. Sure bonds fluctuates some with interest rates, but it's nothing in comparison to stock market volatility. Total bond has only had one year with a negative return in over 15 years and that was 2013. 2013 was another year people were certain interest rates were going to shoot up because the of the Fed. Total bond was down 2.15%, not exactly a crisis. Turns out the markets were completely wrong, and 2014 ended up being a great year to be in bonds when people realized interest rates were barely moving. Predicting interest rates is about as hard as predicting the stock market.
Disclaimer: there is a certain amount of discipline that comes with being 100% stocks. If you don't have it then being 100% stocks could be bad for your wealth.