What is the prescribed way to handle indefinite boundaries? Case in point, unitarian universalist. They might call themselves 'Christan', but no christians I know would.
There are two different issues in the question as being discussed.
First, there are groups that have little defined structure, may not even be orthodox, but are non-traditional churches which still hold the core tenets of Christian faith. These can be discussed as groups on the basis of what they do define. There could be no other discussion, clearly, as anything beyond that would not be pertaining to the group as a whole. There can be no group which defines nothing. Even if unspoken, it would be 'tohu vo bahu' (formless and void, Gen 1:2). Nature abhors a vacuum, and it would either create rules, spoken or unspoken, or collapse in on itself.
But the second issue here is the churches which violate even the basic fundamentals, which most Christians would reject fellowship with. UU churches are not considered Christian by any I know, nor are mormon (im talking about in the eyes of Christians I know), as are others. Further, there are groups which are rapidly departing orthodoxy. Whether or not they have left, though, is a truth statement. But, many groups (thinking of those like Rob Bell), have left many fundamentals, and dont seem to want to have anything to do with what is commonly called Christanity, except in name only. How far will they go?
I am not making a 'truth' statement here, but whether his or another group has gone fully beyond the limits of 'Christianity', the point is (a) there is a limit, and (b) some people have and are going beyond it. And, whether they have or not is a truth statement.
If someone came in saying they were Christians but only keeps the basics of the faith, that is one thing. But, when the fundamentals of a group, by whatever name, contradict the definition of Christian (to most, that is, as it is 'subjective'), to the point of onerecent question asking if Jesus of Nazereth was just a goid idea later written down (not a person), what prevents a site such as this from degrading into to the same chaos as Genesis 1:2, above? If it is simply up-and-down votes, its in trouble!
It's one thing to simply be unorthodox. Its another entirely when people begin to ask questions further and further out. In all honesty, it seems if there was a "universalism.SE", you could direct the later group of questions there, while focusing discussion of the former on what such groups do or do not define, and how that compares to what is written in the Bible or how it compares to orthodoxy (both non-subjective, until intrpretation and opinion enters). But, doing so, of course, would introduce a value decision of its own, wouldnt it? (Which, of course, is fundamental to orthodoxy that it do so).
As a case in point, a ministrer I know and myself have his decision that he was no longer going to agree with the Bible as an authority. He had made some interesting claims already, to which I disagreed, and, when he finally went public with the notion that he no longer considered the Bible authoritative, I was relieved, because, now, he is public that he considers his judgment of greater worth than the Bible (take it or leave it, its what he did). Because of that, I feel relieved, because now, at least, there is no more confusion. He is no longer saying this is what the Bible teaches, he is willingly putting himself as the authority (right or wrong), and there is now no further confusion about his teaching and orthodoxy. He has simlply left it, and now agrees, the thing had insisted all along.