My 2 cents about the whole discussion:
There are a ton of reasons why people have private trees and they should be respected. This is a paid service and If people want to pay to use it but for whatever reason, don't want to share their info, that's their prerogative. Sometimes it's frustrating but it's their right.

1) I post lots of photos cause I want them in my (mostly public) trees and I want other people to have access to them.
2) I have private trees for 2 reasons:

--The info in that tree is of a very speculative, theoretical nature that I am using to try to work out issues in a public tree. I make it private because I don't want anyone using the (probably incorrect) research.
--The info in that tree might be hurtful to people. For example, I recently used Ancestry to track down my birth family. Before I met them, I created a private tree to assist me in my research. I didn't make it public because I didn't want any family members on Ancestry seeing my info and calling up my birth family members to say, "Did you know your mother gave birth to a child out of wedlock in the 60s?"