It's rarely been a better time for borrowers with good credit to tap into the bond market. In fact, the same is true for borrowers with dodgy credit.

It's not just U.S. Treasuries that are seeing near-historic-low yields. Once-radioactive sovereign markets like Greece, high-tax U.S. states like New York and California, and corporate borrowers from investment grade to junk are being rewarded with yields that are barely above benchmarks.

With the record-long U.S. economic expansion on course to enter a 12th year, a swollen supply of investor capital flooding into markets is shrinking or eliminating risk premiums throughout the fixed-income landscape. It's not that investors are oblivious to the eye-popping valuations. In fact, "bond bubble pops" was the second-most-cited risk in Bank of America's latest survey of fund managers. Regardless, many investors ask: What choice do they have but to keep on buying?