The insurance is a gimmick. Buying X bonds with 25% insurance at Y BTC each is equivalent to buying 0.75X uninsured bonds at (4Y/3 - 32/75) BTC each. This means that, all else being equal, buying X PPT bonds at more than 1.07 BTC each is dominated by investing 0.75X BTC in pirate at the same MPR.
The bonds in general are good for people who can't invest in Pirate at all, or too little to enjoy the higher interest rates; but assuming everyone is rational, the insurance shuffles some numbers around but it ultimately has no effect on who should buy them and what they gain for it.
Ah, interesting. I was just intuitively thinking that a lower-risk asset would be more desirable when I want lower risk, despite the lower return.