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Personally, I will not. While I suppose I do have some minimal concerns, I wouldn't consider the potential threat to be anything close to allowing Dropbox or something like that. Sure there could be something I wouldn't want shared in a Siri request, but come on, a two or three sentence question is going to be limited in its security impact. I suppose all of a single user's "Siri talk" could be aggregated, and a fuller picture given of something, but that is a little bit of a stretch in my opinion. If I was working with extremely sensitive data, I would probably take a more conservative approach towards it and block it.
I think that Apple should be more transparent about how long the data is stored and how it is used, and as jimlynch said, they should stop doing this when it isn't in beta. I can cut them some slack while Siri is in beta, but Apple should make it clear what they are doing with your data.