What a marvelous discussion I've read here. I'm figuring out that couple of things are going on when someone privatizes a tree.
Examples: There has been an adoption, or an illegitimate child, perhaps a disputed lineage, or marital affairs that the tree owner doesn't want public.
Or maybe Private Tree Owner is concerned that Tree Owner I am Everything will heist Private Tree's bona fide information, and bastardize that information into I Am Everything's poorly sourced and terribly linked tree. Use their photos, misrepresent information. Or maybe Private Tree's info is still in work, and not certain, and so it's worthless for others.
Personally, I presume my trees ARE a work in progress. There is always a new change, some source that I thought was valid that turned out not to be. There could be adoptions I don't know of, affairs still too well hidden and children yet unknown. Since all trees' living folks are hidden, it seems to me that most people's recent history is still more or less protected. As time passes, the personal flaws of our ancestors can sure explain a LOT. I see no shame in revealing those flaws, particularly generations later! In my paternal grandmother's line, her father was the family black sheep. As such, he left southern Georgia, and so our line lost the connections. What I've discovered by delving deep has been restorative.
If I can see someone's private tree, I might notice that their ancestors lived in the same area as mine. I might find that link they cannot because they might have rejected a portion or thread of their line because their own family history has done so to protect others in their family.
Our mutual ancestor might have been an upstanding citizen, while my ancestor (private tree's aunt or uncle) might have been written out of the family tree. Hiding a tree makes if more difficult to make sense of what's been lost. And makes it more difficult to discover.
Memory Keeper, you said if those with private trees don't respond, that's their loss and mystery, not ours. That's not true. It's a MUTUAL loss. It's an opportunity to find one another.
The poster previous to me said, "I am SO excited about Ancestry's potential because of the huge user base. I can't wait for the day when we're complaining about people not sharing genomes for segment comparison instead of complaining about people not sharing trees. :) "
It IS exciting isn't it?!