If you are a member of Ancestry.com you have the choice of making your family tree public or private. There is great debate among researchers on this question. It is a double edged sword in my opinion, with good reasons for each choice. I personally have my trees public and thought I would share my reasons for this decision.
One reason I have seen stated for keeping a tree private many times in forums and articles is researchers do not want people to "steal" their research. They feel they have worked long and hard to gather the facts in their tree and others should not "get it for free". I would think this is especially true if they have gathered many sources from outside Ancestry.com not readily available. I understand this feeling especially if you have traveled far distances and spent money on ordering records. I personally enjoy sharing rare finds. Actually I am rather proud when I find an obscure record or source. But not everyone enjoys sharing so it is a personal choice.
The biggest drawback in my opinion with public trees is people taking your information and using it incorrectly. I have seen this happen many times and it has happened to me. They attach records to the wrong ancestor. If your ancestor has a common name there could be hundreds if not more people out there with the same name. This causes a snowball effect of incorrect information getting added to more and more trees because people assume it is correct because they see it in so many places. While this is a concern of mine I cannot control what others do and I feel for my purposes the benefits for having my tree public outweigh the downsides of people using information incorrectly.
If you attach "sensitive" information to your ancestors that you do not want others to see then it is smart to keep your tree private. Many do not want others to see out of wedlock births, suicides, ancestors in jail, etc. I do have a few cases in my tree where this applies to more recent ancestors meaning there are people still alive who are connected to the sensitive information. In these cases I do not attached the sensitive facts to these people on Ancestry. I keep all that information in my genealogy software program. You can also add Notes to people in your tree. These Notes cannot be seen by others even if your tree is public unless you give someone access to your tree as Editor.
Many researchers will create "test" trees for data that is not proven. This can be helpful in finding more information on the ancestors in question because you never know what will come up in the Ancestry hints. If you have a tree that is purely a test and likely contains errors keeping it private will prevent people from using it incorrectly. I only have two trees, one for my own ancestors and one for my husband's. I do have a few cases where I am almost certain a family is connected to mine but not sure how. I will add these people to my tree but only with proven sourced facts about them--I do not tie them in to the rest of my ancestors. For example if I believe "John Smith" is my great grandfather's brother I will add him to my tree with his spouse and children but would not link him to my great grandfather.
Many who add photographs to their tree keep it private so no one can take and use their pictures. It is a fact of the internet that if you post a picture people can and in many cases will--copy and use it. I love finding pictures of my ancestors and have been able to identify some of my own photos of ancestors from pictures I have found on Ancestry. I do post many of my pictures and enjoy sharing with others. The main problem with pictures and again this has happened to me is someone will attach them to the wrong person. This does bother me and I usually send a nice message to the person stating why I believe they have my photo attached to the wrong person but that is no guarantee they will correct it.
So with all these drawbacks of a a public tree why do I make mine public? For myself the benefits outweigh the negatives. My main reason is being able to connect with others researching my ancestors. I have found numerous "cousins" through my ancestry.com trees. A few of those have become friends who I have been able to meet in person. I have been able to learn so much more about a few of my ancestors from making these connections. I love to find details about ancestors that just are not available in traditional sources. Family stories, pictures, details that are not in records but have been passed down through generations -- these are things that sometimes can only be found by sharing with others.
While I am careful to verify any records and information before I add them to my trees I do sometimes make mistakes. I have been fortunate to have others find these errors and send me a message so I can correct them. This would not have happened if my trees were private.
In summary each person is responsible for the accuracy of the information in their trees. There are so many types of researchers on ancestry.com. It ranges from those serious about their information providing sources for all their facts to basically name collectors who will attach any record they find or that pops up in hints without verifying or stopping to think if the information makes sense. I will continue to make mine public because I have gained so much through my connections with others that would not have happened without my information being public.