This is a good reason for all of us to keep switching away from GMail. It benefits us all via network effects.
I've wondered about this from time to time. Its one thing to scan emails to serve ads to the person who is using Gmail, but is Google also using those emails to also create profiles of non-Gmail users? I would like to think it's not.
It's likely that they do. It seems to me like it's hugely valuable and I see no reason why Google would refrain from doing so. Plus, Facebook does it too.