Well, the theory is that the volatility oin your bonds is good beause bonds tend to zig when equities and other things zag. Naturally in periods of extreme stress, correlations all go toward 1 and diversification benefits become hogwash.
I had my parents sell a big wad of appreciated corporates and put the proceeds in the PF 5% deal. The attraction was as much about lowering risk for a very small yield give-up as it was about converting ordinary income (~10% all in yield) to cap gains.
As I was doing some analysis for Ralph on the couch potato portfolio I was struck by how consistent the performance of the VG Bond index fund was.
The 5, 10, and 15 year are 5.72%, 5.57, and 5.83% respectively. Now individual years within the last 15 have varied more than that 2.4 to 11%.
Vanguard GNMA returns are about 30 BP higher
Thinking back it has been relatively easy to lock in 5+ year CDs from PenFed and others at 5,6, and even briefly 7% over the last decade.
Importantly a Penfed 6% CD outperformed the Total bond market fund during 2008. 6% vs 5%.
CD's offer some pretty compelling risk protection. If interest fall that can't be called, if interest rates rise you can pay the early maturity penalty and refi them. Oh and they are insured by what appears to be a mostly competent regulatory agency.
Now maybe I am cherry picking the data here, but if you aren't Brewer and aren't able to intelligently evaluate the bond market why not use CDs instead of bond funds.?