Parasocial relationships are not bad per se. Let's say you are thinking about Donald Knuth when working on a computer science problem, nothing bad here, taking inspiration from the leaders in the field. But it is also a parasocial relationship, it is like imagining Don Knuth next to you, helping you solve your problem, even though he has absolutely no idea about who you are. It is a one way connection, but here, it is actually productive.
If you’re ignoring the “believe you have a two way relationship” then everything could be defined as parasocial.
It feels like it is two-way, in other words, it is an illusion, but just like with optical illusions, you don't have to believe them. For example, mirages may look a lot like water, but people who are familiar with them know it is just a trick of their senses and don't assume there is water there. Same thing for parasocial relationships, even the most intense. Proof is, parasocial relationships with fictional characters is common, and most people who feel a bound with Harry Potter are not crazy enough to believe the feelings are shared, as they are aware that Harry Potter doesn't actually exist.
And yes, I believe that parasocial relationships are extremely common and in most case, positive or at least harmless. I don't believe reading biographies is always parasocial though, it could just be the search for academic knowledge, without any feeling of connection, but done repeatedly, in can become one, which is again, not necessarily a bad thing.
You can absolutely have a parasocial relationship with Anne Boleyn, and I suspect most people who study her in depth do, as picturing oneself with her can help better understand her life and its historical context. It is essentially a mind hack, instead of just using logic, you also use emotions.