I have a number of trees uploaded to ancestry, most are public and a few are private -- private only for the fact that they are theoretical trees I am working on and do not want people to take the 'mis'information and post it all over the internet. When I see a 'private' tree I assume they have good reason to keep it private.
I have had hundreds of people 'attach' my photos and documents, I have also 'attached' many from others trees. This should be about sharing. Who would want my photos or documents except kinfolk? The reason I upload my documents and photos is there are soooooo many trees with wrong parents, wrong spouses, siblings, etc... I can only hope that my documentation will help my 'cousins' correct their mistakes.