I came here looking for an answer to this same question. Here's why I would want unrelated people. My paternal grandmother was about to marry the great love of her life (or so she thought) but then he got another girl pregnant, a shotgun came into play, and the next thing we heard on the subject was when she gloated in her diary about how he had died after a lingering, painful illness. Strictly for my own amusement, I started searching. I actually found him in the 1900 census, and need a place to make notes on what I can find out about him and his family. Thus the desire for an unrelated persons tree.
Edited later to add, in trying to track down someone who is in my family tree, I have discovered that findagrave has no less than 8 people with the exact same name (and it's not something you'd think was a terribly common name, either), and 2 of them died the same year. And NONE of those is my relative-by-marriage. So as I track each one down and sort out where they do (or do not) fit into my family tree, it would be REALLY nice to be able to document the results of my research.