I get what you're saying about the subjectivity, though. While I was trying to work out my type, it struck me that people differ greatly in their conceptions of any given type. And it's difficult to get objective advice, because, for an example from my own experience, INTJs don't really 'get' INTPs, and vice versa. There's no one source of information that can tell you what one type is like and what the other type is like with the same degree of authority, objectivity and insight.
Is one of the reasons I'm not entirely convinced of the validity of MBTI in its current form. At least, there is room for middle-ground people who fall between types, and there is a lot of variation within types, because there aren't sixteen neat kinds of people in the world. Type must be, at best, an approximation, to which some people may happen to more closely conform than others.