There are reasons other than a reluctance to share for people to have a private tree.
My trees are private because I don't want ancestry.com to put them up on mundia.com (which they do to the public trees). Most people willing to pay to put their tree on ancestry.com are serious about genealogy. I don't mind sharing. However, I don't want all the information contained in my tree available to every person with access to the internet.
Crosby/Waller Family Tree