My thoughts on private trees that are DNA linked - I too have found it very frustrating/irritating when I have DNA matches show up as private trees. I do understand the reasoning some have for keeping it private but why can't Ancestry at least show us what the common surnames are between two DNA matched trees so that I would know what to contact the tree owner about and whether I want to bother? I do have some particular surnames I would want to follow up on and others that I would not.
As for sharing photos - I just want to share that I have quite a few extremely old photos that I puzzled for many years who they might be. From sharing here on Ancestry I have had great luck just in the last year of Identifying some.
As for junk trees - I agree - I have looked at too many trees where the owner doesn't seem to notice that a supposed mother was only 5 years old when her children were born! LOL