Unfortunately, Siri has no loyalty - if someone else gets possession of your phone, Siri will obligingly read them your texts or e-mails - or send text and e-mails that appear to come from you. This is true EVEN if you have your phone locked with a PIN.
This recently discovered security flaw can be corrected, but you must take the affirmative step of disabling Siri when the phone is locked - and how many users are going to do that? Unless you take that step, be wary of what you share with the faithless Siri!