'Openness' amongst a community or society and openness with regard to the government are not one in the same.
http://en.wikipedia.org/wiki/Fourth_Amendment_to_the_United_...
I think we should encourage people to come forward of their own free will, both by making it safer to do so and by educating them on the benefits to society. In a sense I think this because I have faith in my local police, and see them as a part of the community, not as an "other."
Same as you, though, I don't believe believe in expanding their powers to compel people to come forward, because that obviously has scary negative consequences.
While I generally support unions, the police fraternity mindset is troubling. There are no good cops as long as the blue code of silence is the de facto mode of operation.
My personal view is that rights are most greatly abridged in crises, and that the voluntary cooperation is less likely to beget oppression than is the unrest and crime prevented by cooperation. Moreover, I think a culture of forcibly requiring information is more likely to arise in a culture of reticence.
You're right that it's a difficult line to walk, though -- defending a cultural value of openness and strong rights to silence and privacy is a subtle point.