People, including genealogy researchers, are not perfect. Well, maybe there are some who are:) but I am not one of them. I have on two or three occasions had a need to merge two individuals in the same tree. Usually this was because the same individual used two different names on different enumerations and I did not catch it!

I make it a practice not to use the trees of others as they are often inaccurate.

My own genealogy software, Legacy, makes a provision for this. I agree that it would be a useful feature at ancestry.