So I cast my vote, in isolation or with others, in favour of Ancestry Public Trees. I accept however that as Peter -no lover of Ancestry Trees Public or otherwise- often advises, those troubled by the apparent inanity of ‘copy-cat’ researchers should make their Trees Private.
I say there are really five types of Ancestry.com tree users:
Ancestry Public Tree - Don't Care - these folks will not return a kindly message, say their work is "done" and that corrections are your problem. These are the rudest people out there and usually send lots of nasty messages about your tree not being "open" and that you should willingly donate whatever you have spend thousands of dollars on. Date last worked on their tree is usually years ago. Tend to have the most mistakes because their sources are "Ancestry Tree" "One World Tree" "Millennium Project" and "Family Data File on Ancestry".
Ancestry Public Tree - Don't Know - they have no idea that their family tree maker uploaded a tree there and have no idea how the system works. They may be interested in fixing problems or adding more data but are in genuine need of education before they can do anything. Often easy to spot by the date they last worked on their tree - its often abandoned from years ago.
Ancestry Public Tree - Public Folk - these folks are interested in public contact and want to help others with their research. They return messages or emails and want to talk about things and make ongoing changes to their tree.
Ancestry Private Tree/still listed in search - these folks are interested in public contact but want to chat with people before allowing access to their information. Count me in this group - I have met way more researchers in this mode than I ever have in public mode and the more researchers I meet, the more that I see do a ton of work and want some element of control over the information that goes out with their name on it.
Ancestry Private Tree/not listed in search - these folks are interested in confidential research for whatever reason. Often they are heir hunters or professional genealogists working on something for a client. They never return messages or other contact about the person.
There have always been name collectors and folks interested in the "Bible-type genealogies" (i.e. "Bob begat Simon who begat John with bare minimum names and sometimes dates) and folks who genuinely don't give a hoot about the veracity of their research. Unfortunately, I think Ancestry's "magic leaf" commercials and the US version of WDYTYA have only led to the idea that faster than you can say "Abracadabra", your genealogy is available online and complete and so it takes a lot more work for people to understand that there's a lot of work, money, and talking to people involved in this hobby to really get somewhere on your tree (I won't say "finished" because no one is ever finished, really).