It does simplify recovery after a Heartbleed-like bug, as described in the blog post. Of course it doesn't prevent the bug from happening, but it does greatly simplify recovery afterwards, which is important to me. The security properties are in fact the most exciting thing about short-lived certificates for me personally.
For the sake of transparency, there is another motivation: For a 1-week certificate, we pay 1/52 of what we would have for a 1-year certificate. So if a user installs Sandstorm, gets a certificate, and immediately uninstalls, we don't pay for a full year. More importantly, if a user installs sandstorm several times -- perhaps due to a bug or a misunderstanding -- and gets a new cert every time, we don't take a huge hit. Basically, we get some risk management out of this deal.