Well, first of all, "breaking in" isn't the only way someone might get access to data on Google's servers. There are such thing's as subpoenas, not to mention that it is possible a Google employee might abuse access to servers. And then I would be _very_ surprised if google doesn't use the content of your emails for advertisement and tracking purposes.

Furthermore, unless both parties are using gmail, the email will be stored at least temporarily on other mail servers, which may be less secure (and you might not even know who controls them).