To me it is 100% about functionality.
I live with one female who has a confirmed diagnosis of DID. The other has a confirmed diagnosis of BPD.
These are pretty serious illnesses, however both my partners are fully functional and able to have loving and lasting relationships, full time jobs, go to school, raise children, etc.... Both have dealt with this through countless years of therapy before and during my time with them.
Things are more stable now for us then they were ever when we were apart.
Granted I am NOT a caretaker and have no desire to be one, but working with and around mental illnesses is doable for me, and having a love who has DID is almost like a poly relationship in itself.