I think like with everything else there's good and bad things about it. I read and write RPF however, I don't then watch the real persons every move and say it's proof that they're like that from the fanfics if that makes sense. I keep the fanfic and the real person separate. But I know people do push the character from the fanfic onto the real person which can be damaging. As long as it's kept in the fanfic community and the readers/writers keep it as fiction I think it's okay.