One thing to remember about Wikitree is that we share common ancestors on a common tree. So merging 'one of their ancestors with mine' basically means that there are two profiles for the same person, and this isn't acceptable. None of us 'own' a profile. We can 'control' profiles of living people, but not of older ones. Anyone can go in and change the data within a profile (unless it's a closed profile). The good thing is that the changes are recorded, and if someone erased something you wanted, you could go back and restore the data.