It is strange how we all follow the same pastime but we view it differently.
I don't see it as collecting ancestors which I then possess, I see it as shedding light on my ancestors. The great and the good (and the bad) will always have their history's written, we are writing the histories of the unwashed, unlettered and up til now forgotten people. I publish this on Ancestry because I want others to see it, copy it, use it, extend it. I want others to tell me if they think it's wrong, I want to (and have done many times over the years) collaborate with others to improve my family's history. I have seen connections that I have found after years of looking (intermittently) get copied and begin to outweigh the nonsense trees on Ancestry. None of this could happen if my tree wasn't public.
I think of what I am doing as like the child spending years researching and writing the autobiography of their famous parents, I want to publish, I want to show the world these people and I want it to understand their lives.