If you don't want people to know that you're friends with Dave, don't friend Dave, or don't use it at all. This isn't supposed to replace pseudo-anonymous networks like Twitter, this is more like Facebook without all the bullshit. The use case is "I know Dave, and I don't care who knows that I'm friends with Dave, and I'd like to let him know I won't make it to the PTA meeting without involving an enormous advertising corporation".

The fact of the "vouch" could provide the validation without identifying the people from whose circle overlap it was derived.

I think the point is that you wouldn't be. The network, from your perspective, will be quite small.