Another from the "if the data exists, it will get compromised" file.
This article from the Washington Post talks about an interesting case of counter surveillance hacking.
In 2010, Google disclosed that Chinese hackers breached Google's servers. What only recently came to light was that one of the things compromised was a database containing information about government requests for email records.
Former government officials speculate that they may have been looking for indications of which of their agents had been discovered. If there were records of US government requests for information on any of their agents, it would be evidence that those agents had been exposed. This would allow the Chinese to shut down operations to prevent further exposure and to get those agents out of the country before they could be picked up.
I had not thought about subpoenas and national security letters being a counter intelligence treasure trove, but it makes perfect sense.
Because Google / Gmail are so widely used, they present a huge and valuable target for attackers. Good information on almost any target is likely to live within their databases.