I have a very popular first name, but it's still way less than 1% of population for my gender and it's already "too common" at my workplace such that we have to use surnames instead
I've also met people who – against the normal, mainstream Japanese way – basically always prefer their first names, because their last name is so common people are going to mix them up with... someone. I sometimes sense a kind of an aura from these people: they don't want to be seen as just "one of the Satо̄s or Tanakas". (Btw. those are the exact names of the people that came to mind, whom I felt that aura of not wanting to use their last name.)
I think that those kind of people would choose the rarer name for their kids. But they don't also want to change theirs, because they are in the middle of a professional carrier, and being the busy people they are, they don't want to stir things up.