Google is using the content of e-mails for its own good, that much is true. It is a well-known fact that e-mails are scanned for certain content, in order to bring the user more relevant advertising. The other features mentioned here use the scanning as well, although I see no reason why it couldn't be done client--side. As long as no human ever gets to read the content of what's in those e-mails, and more importantly no harm can come to the user because of this scanning, all is well. That said, if you are sending anything sensitive, other platforms are far more appropriate.