I'm always surprised when investors are concerned about the bond side of their portfolio. The bonds are the safety net. They are boring, even when they have a bad year, they are really really boring. You know what a bad year for Total Bond looks like? -2.15% That's the worst year it's had in the past 15 years, and it's the only time it's had a negative return in the past 15 years.*
A few items to consider.
1. We don't really know what interest rates will do. In 2013 investors were so sure interest rates were going to rise that they sold out of bonds, that's the year Total bond was down 2.15%, but they were wrong. Interest rates went down in 2014, not up, and 2014 was a great year for bonds.
2. Total bond is diversified across maturities, short, intermediate, and long. That's why it was down 2.15% in 2013 when many long term bond funds were down over 10%. Total Bond also managed to maintain positive returns from 2003-2006, which was the last time we experienced sustained rising interest rates. Even if rates rise it could still have positive returns. A slow rise in rates isn't a serious threat, it's priced in. It's the big unforeseen jumps in rates that pose a threat, and those are normally due to inflation which is nasty for even more reasons.
3. This is the important part. This is the primary reason you add bonds to a portfolio that already contains stocks. When stocks go down bonds tend to remain stable, OR even rise in value. When stocks go through a serious crash we experience a flight to quality, a.k.a a flight to government bonds. Total bond was UP 5.15% in 2008. That gives you two very powerful tools for portfolio management. It allows you to control how much risk you are taking, and it makes rebalancing stronger. Cash, CDs, money markets, and savings bonds don't act as the same hedge against stocks.
The consequence of 3 is that a stock/bond portfolio is more efficient than a stock/cash portfolio. In most years bonds earn more than cash, and bonds act as a better hedge against stocks. Therefor, a stock/bond portfolio will normally have higher returns and less risk than a stock/cash portfolio.
* I picked the past 15 years because that is what I could find on the website. Going back further, the worst year for a balanced bond portfolio(pre-Total bond) was about -8%. That's still very little risk relative to stocks, and that drop was due to hyperinflation.