The issue with private trees is that the major value of autosomal testing is in comparing possible cousin matches and sharing information. The private tree people can see our matches but deny us the chance to find potentially valuable data from their trees.
I think that a good compromise would be to have a surname search function that includes private trees, so we could look for surnames we are specifically researching. I have lines I have matched a dozen times and am not concerned with another match. I wouldn't bother to ask for permission to see that tree. I also have lines I am still trying to prove and every possible match is valuable.