Some girl had lost her iPhone god knows how. All I know is that I found it on the side of the road near my home, where it apparently had been for 2 (two!!!) weeks during the winter. It had snowed twice and rained once, not to mention the salt runoff. Long story short, it turned on after a rice bath and I was able to call her parents on her own phone using Siri. She got it back about 6 hours after it had turned on.
You overestimate Siri if you think leaving it on will actually allow people to steal all your information. It did not allow me to look at her contact list, and I only found out the name of her parents because her last name was on the home screen itself. It DID make returning it more convenient though.
Seriously, I spent a few minutes first trying to get Siri to "call home", or "call mom". The far larger breach of security was the text list on open display. Worry about that first if you're paranoid. Also: how the hell does a cell phone wind up on the side of the road without you knowing about it?