I think the more important distinction to make is between compelled testimony and voluntary testimony.

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.