Your points about those who might be constructing trees and keeping them private due to non-parental events are valid ones that I didn't consider myself when I first received my test results. I have known of cases in which someone underwent DNA testing due to their suspicion that "Dad" or "Mom" were really not. If they want to continue their post-testing investigation in private, who are we to judge them or to criticize them for it?

To anyone who has just received their initial results and is getting frustrated when coming across a private tree, rest assured that you will likely soon receive more than enough matches with public trees to keep you busy and you won't lose any sleep over those matches with private trees.