ETF portfolio and long term index investing research including my personal research and bogleheads / vanguard research:
Do you want to buy cheap Equities? Holding Bonds may precisely give you that opportunity through re-balancing. Unless you can put your cash aside for 10+ Years you may want to check how Bond ETFs eg. iShares AGG can increase your returns and reduce risk of high losses.
Here is a simple comparison of 100% S&P 500 Portfolio vs. the most commonly used 60% S&P 500 40% Bonds Portfolio