The Ignorant Investor

NASDAQ has an article up at its site that notes:

Treasury bonds and treasury bond ETFs were among the very few assets that did not decline during the worst moments of the financial crisis in 2008-2009. Even gold and silver performed erratically during this period. U.S. treasury assets proved to be negatively correlated with the broad market. As equity benchmarks were cut in half, treasury bonds ETFs soared to new, all-time highs

I read a book by money manager Robert Swenson a couple of years ago that pointed out the reason for owning U.S. treasury assets. It's not for the return. If you want high return, you go with equities or equity-like assets. Instead you buy treasuries to diversify, so that when equities start going south in a panic, there's a counter-weight from treasuries that boost bond returns during the frightening times when portfolio values are dropping.
He pointed out in his book that people will often go with corporate bonds for the extra couple of percentage points in yield they offer, but since the ability for corporations to pay bonds is highly correlated with their profits. Corporate bonds tend to do well in good times, and not so well in bad times. So if you're optimistic enough about the economy, you may as well buy equities.
Swenson might be overstating the case, but it's something to think about. Comparing Vanguard's Total Bond Market Index over 1,3,5 and 10 year periods indicates that Long Term Corporate bonds might out or under perform the broad market at times, it all equals out in the end. More or less.