The most popular database on Ancestry.com is the Public Member Tree with over 2,100,000,000 records which are more entries than any individual US Census. That is enough to make one concerned about the value of a member tree. To those new to research, finding another person's family tree is like a kid being let loose in a candy store. What the more seasoned researcher knows is this: A family tree with no sources is like a diet with only sugar and no substance to sustain life. A tree with no sources ought to have a flashing red sign: "Researcher Beware!"
Trees with no sources
Finding trees are relatively easier than taking the extra effort to dig deeper or order a resource that is not online. It may even be thrilling to find someone else that has your ancestor listed on their tree, but what good is finding your ancestor on a tree with no proof of family relationships? Taking unproven information into your tree is like scooting out on the weakest limb.
It does not take much time to move back adopting ancestors using undocumented trees, but you are sure to reach the end of the line ancestor somewhere. When there is no tree to document the next generation, people usually are forced to consult historical records. This is when the weak limb breaks because nothing jives in the records. You are forced to start over with what you can document as being true.
Brick walls just do not show up in your path. You build them around your ancestor by following undocumented paths. If you have climbed your tree a few generations back without solid sources, you risk placing the wrong ancestor in the tree and wasting a lot of time.
Here is an example of research results achieved using documentation from FamilySearch.org:
Catherine Eliza Mallet was born in 1849 in Nova Scotia according to the 1900 Census. According to the 1900 Census, she immigrated in 1863. She married John F. Bushey of Canada, and in 1870 they lived with their children in Essex County, Massachusetts. By 1880, she was widowed and lived with her children in Gloucester, Massachusetts. She and John had the following children:
Sarah E. Bushey
John F. Bushey
Mary L Bushey
Augustus Ildegarde Bushey
Stephen M Bushey
Now compare the information above to another tree that had no sources attached:
Catherine Eliza Bushey is listed as deceased in 1880 although she appears on the 1900 Census in Beverly, Massachusetts. The following children are listed: John W. Bushey
John W. Bushey (wrong middle initial)
Mary Isabell Bushey
Stephen M Bushey
Augustus Frank Bushey (wrong middle name)
If research in the second example was done using resources, the researcher may have noticed that the 1900 Census documenting Catherine was overlooked. A lot more details were gleaned from using records than from relying on trees. These records open up other avenues of research and provide more information about Catherine's life. This is not to say all family trees are erroneous. Just take the ones with no resources with a grain of salt.