As has been said elsewhere, this wouldn't work, since the client has to somehow decrypt the file in order to use it. Decryption => a decrypted copy is stored in RAM => a clever program can find and copy it.

So this would be rather dangerous, since it would give users a false sense of security, prompting them to be even more careless. I know lots of users are whining right now and blaming the devs for not including encryption, but this is simply the truth of the matter.