Re: "I know that you don't "create friendships" by unimpeded data import from another tree. You do that by personal communication. A public tree owner is no more likely to respond to a query than is a private tree owner...arguably even less so."
On the responses, you may well be right. A public tree owner may, though, be far more likely to RECEIVE an initial contact request from Burgess Donnelly than a private tree owner, due to quite reasonable concerns with efficient allocation of time.
I do not maintain a tree on Ancestry.com, so I care not a whit about importing anyone's data. But what shows up on the search results list for matches that occur in private trees typically does not give enough information to be able to tell if the tree owner is a person I would be interested in contacting or not.
Looking at a public tree often allows me to ascertain whether our research interests are likely to overlap. Often examining the tree allows me to tell that the match was partial, and the hit was for an unrelated person. Even when it is the SAME person I'm researching, I typically do not bother making contact if we each know the same info and are unlikely to be able to help each other, or if our interests come in from sufficiently different collateral angles.
There are usually WAY too many search hits in private trees for me to bother sending 50 tentative contact messages just to ask if the hit might be someone I'm interested in. (Or in making lots of detailed related searches to scope out who else may be in their tree - that also takes too much time to bother with.) Also, about 95% of my contacts are initiated by way of my correcting information in someone else's tree or providing additional information to them. Without examining their tree, I do not know if they have errors or missing info that I might be able to provide.
So it is very rare that I bother contacting the owner of a private tree. The likelihood that our research interests do not significantly overlap is usually too high to bother with. I typically just treat hits in private trees as black boxes, and ignore them. I probably initiate contact with owners of public trees 20 to 30 times as often as I do owners of private trees. Burgess may have a similar communication pattern.
Note that I am NOT complaining about people making their trees private! It is totally fine with me; I am not hurting for contacts; I have more than I can keep up with anyway. But I do think you are overreaching to assume that someone ELSE does not know their own research patterns well enough to have a good idea as to whether the public vs private standing of a tree makes a significant difference in the likelihood of their making useful contacts.