Matt Levine: If you wanted to be a jerk, you could buy $1000 of Greek bonds and $1000 of CDS and wait. The voluntary exchange would happen, you’d ignore it, and your debt would come due. Then you’d hand it in and expect your 100 cents on the dollar. If Greece honored your debt, you’d get 100 cents on the dollar on the debt and zero on the CDS. If it didn’t, then that definitely would be a credit event, and you’d get back X on the debt and 100 – X on the CDS. It is not entirely easy to predict which of these things would happen: if this purported exchange (or a future iteration of it) goes very well and leaves only a little stub of old debt, Greece will probably honor it; if not so much, then you’ll be looking to your CDS. Because you have no idea which will happen, you buy both.
All that is obvious. But you have to be a jerk to do it, because think of all the Greeks who will face ouzo shortages due to your dastardly market manipulations. And in particular, you probably have to not be a big bank.