I've had Apple phones since 2007, and I've used Siri about three times. And I think it's a very poor idea to let your phone spend money without asking you if it's okay.
How about sending everything you say to apple servers to be 'analyzed'. I've never trusted the cloud concept and never will.
If there is anything sent off my devices and computer to a cloud or off site server, it is without my permission. The only server I do that with is blizzard, and who knows what they do with data.
And at 100K a bitcoin, you can guarantee some site or game server is mining as well.