Email Security around forever – just turn it on

I read this article by George Ou and I’m confused about his statement “The problem is that AT&T doesn?t disable unencrypted mode, which means the vast majority of users won?t use the secure transport mode.”

I don’t understand what this means. Should I not bother to turn it on because AT&T doesn’t disable unencrypted mode? How does that affect me if I turn it on?

Thank you.