It does propose an enormous gray area, but not one which is unprecedented.
Workplace.SE is the first one that comes to my mind. Even Software Engineering has many answers that are not completely backed up by hard facts.
Because questions will be dealing with interactions between individuals in differing contexts, as opposed to documentable facts & events, subjectivity is both inevitable and appropriate. This does not mean, however, that there can be no theoretical foundation or facts used in answers; I suspect that statistics & scholarly theories will be cited regularly, though not always.
Ultimately, an accepted answer will be one that the OP finds the most useful. If I were to ask a question about interpersonal skills, I would expect a valuable answer to be subjective, experience-based, and possibly sprinkled with some data. Essentially, we're talking about human beings, who are full of nuance, and so subjective answers are likely to provide more value(and will be in more abundance) than strictly hard data.
If no factual events or theoretical foundations were ever used on Interpersonal Skills to provide answers, I see no problem with that. I would personally be a little disappointed, but it's not really a problem. The exception would be if an answer is too brief, lacking detail, or is considered unsound by a reasonable person.