You're coming at this from a "hedging" perspective-- Instead, think in terms of what the bond gives him-- a lot of Yen. That's opportunities to invest. The increased "risk" in this sense is miniscule-- since the cost of the oppurtunity is less than 1% Yen a year, even a 200% rise in Yen/USD only actually costs Berkshire 2%.

And, my bet is Buffett knows of some companies he could deploy that Yen in right now to cover the interest anyway. He's just lying in wait for better oppurtunities.

Now, there could be some other thing going on-- but taxes-wise in general U.S. companies issue US-denominated bonds to repatriate funds so I'm sure where you're going with that. My bet would be if it's not just for the opportunity, it may for their re-insurance reserves.