I think it is a question of delivering the service you are being paid for.

If you are a licensed clinician, and market yourself as such, and someone comes to see you specifically because of that qualification, it's implied that you are being paid to provide professional clinical services. Along with that goes an expectation of confidentiality and a professional level of care which definitely doesn't include socializing with the client outside of session. I have heard quite a few stories of people whose therapy failed (sometimes spectacularly) as a result of a T with poor boundaries and good intentions. Although it might not be harmful to everyone, it can be harmful enough to some people such that licensing boards decided they can revoke your license for that kind of thing.

On the other hand, there are some T's that may offer "personal coaching" services on the side which are not marketed as therapy or as something designed to treat mental illness, and this kind of service doesn't come with the same expectation of confidentiality or firm boundaries. EDIT: In the case of someone offering both types of services, the client pool for one or the other definitely should NOT overlap.