Me to Siri: "add to my calendar Tuesday noon meeting with John"
Siri: "which John?"
...and then you can't proceed without picking a contact from the list. I'm afraid Siri is going to send a calendar invite or something to whichever contact I pick. I try to avoid using names now, or use funny names I know aren't in my contacts. Why does Siri need to know "which John"? Just add it to my calendar!