The one private tree I reached out to (it was 3rd cousin match) was generous enough to grant me access, however, I still was unable to find the link because a) they didn't have a very big tree; 2) it's quite likely that they are actually related to my adopted Dad's birth family. At the time, I didn't have a yDNA match to a surname, like I do now. Since I have all my mother's great grandparents and 75% of her great great grandparents, if they were related through my mom, I should have been able to find the link.

I wouldn't write off all the private trees. I think everyone has a different reason. In fact, if you are looking for a birth family like I am, it may be out of courtesy to a family that was previously unaware of a non-paternal event. My Dad's birth parents were not married, so there's no telling how they hooked up in 1930 and if there's a good chance he has living half-siblings out there that are unaware of their parent's past.

I have a couple of private trees (albeit not linked to my DNA results). One tree is being built with the family my brother's yDNA matched to. Because it is in early stages and I may make mistakes while I research, I've kept it private. I have been contacted by a cousin to this family and we are working together to create a more accurate tree.

Another private tree I have is my "Royalty" tree, because I downloaded a GEDCOM from dubious sources, and am not going to pull all the hints down on obviously public figures. It's just for fun so I can plug it into Family Tree Maker and see how I'm related to QEII. (13th cousin, well-removed - descended from one of James IV's mistresses)

Anyone who gets their nose up in the air due to their Royal connection has not yet learned that if you're European, you are more likely related to Royalty than not. The fun part is discovering HOW you're related, rather than putting on airs because you think you are somehow more special than the unwashed.