Think of it another way. Same way for bonds. The only thing that matters with bonds is that they pay interest and redeem at maturity as promised. Bonds should be used for either of two purposes - to provide income or to preserve capital. Note that I've said nothing about diversification.
Bonds water down a stock portfolio and have no business being there.
When you are at a stage in your life when you need your investments to provide income, bonds are your best choice. Similarly, if you can't afford the fluctuation associated with stocks, bonds lock in your yield through maturity. When creating a bond portfolio, the most important thing to consider after quality and rating is allocating maturities over time. That means spread your money across several bonds, each with a different maturity. This way, a bond is maturing every couple of years and you can reinvest at the then-prevailing rates.