My husband has NEVER had a shaking leaf hint for any of his matches (he tested one year ago). His family emigrated from Ireland and Scotland. When I see a 5th to 8th cousin match, I always wonder if there might be that "missing link" in the other tree that would help us go further back. But when I've sent e-mails to the private trees, I've gotten replies such as "No, I don't see a match," if I even get a reply. And no offer to share. There might be a name in that private tree which would give me a good lead, but I don't get to see it at all.
Since you can mark anyone in your tree as "living", and therefore have them show up as "private" in a public tree, why even have a private tree? As the previous poster suggested, make up a bare bones tree to share if your family information is too important for the rest of us to see.