"a philosophy of sharing everything" - Alec, I question whether or not any of us, including you, actually adhere to this philosophy, or if it is merely an unachievable hypothetical philosophy?
In fact, I would posit that you do not share everything - you only share a subset of what you have to share. You may choose to share what you share with everyone in your network, but you don't share everything in your life. And since you don't share everything, you are already filtering, and if that's the case, why make a distinction between your filtering and the filtering facilitated by the circles feature?
Perhaps your query needs to be restated as: "It's hard to reconcile G+ Circles w/ a philosophy of open access & letting others decide if it's relevant to them or not"
Thus restated, and even as an advocate of open access, I nonetheless think that there remains bits of information which are not best shared in a fully open context, as many others have here commented.
(sorry, you caught me in a metaphysical moment :D )