Plain text email has never been secure, so if someone is concerned about privacy, there are tools like PGP:
http://en.wikipedia.org/wiki/Pretty_Good_Privacy Google scanning mail should probably be made more public, and get more transparently stated, though.
Google is not a charity, even if they like to give that impression. The price for using their services is the information (and analysis of it) that we provide when we use their search engine, mail, etc. It is quite valuable.