I think it's a very strongly held belief that ME and CFS are separate and that the really valid point is about the way that the definitions have been broadened out from a tight and specific set of criteria. It makes far more sense to study the most severe patients and a well-defined core group as a separate entity - the problem comes when a lot of other people have a set of symptoms which seem awfully similar, and there seems to be no obvious place to draw the line and separate the conditions.
I think this "certain ME website" (and I personally see no reason not to name and link any external website when discussing it, even if controversial by the way, unless worried about attracting their attention perhaps...) make some very valid points. I have immune abnormalities, but I haven't had severe neurological symptoms - I've had considerable brain-fog, memory loss, OI, and dizziness, but a great deal of that turned out to be effects of sleep disturbances which I've now got more control over, and my symptoms were a long way from the most severe cases I've read about. It's kind of insulting to suggest that I have the same thing as someone who can't tolerate any light and heat at all, and is profoundly disabled by other neurological symptoms, and I don't have any problem with them saying to me: "What you have is nothing like what I have!" Then again, I've met people who have or who did once have very severe neurological symptoms who are also among the small number of people who also have my own rare, signature, "impossible" symptoms.
So perhaps the problem can be summed up that many people like me see my illness as being in many ways like ME, and perhaps connected to it; and that some people diagnosed with ME also see that connection, whereas others don't...but that we all see the definition of the condition being broadened out so widely that it is very likely catching people with completely unrelated conditions - and so many of them as to completely obscure the reality. And somehow this spreading of the definition has followed the spreading of the epidemic, and each time the goalpost is moved and the pitch is widened, potential understanding is lost. There's a kind of Catch-22 about it all actually...
So I can totally understand why anyone would want to draw a line, and we do need to define subgroups, so I think there is value in the message of this point of view. I am very open to the possibility that they may be dead right, and there are certainly a lot of smart and experienced people in that camp. The problem I often see though, is that a lot of arguments around this subject are about names and labels, and really such things are artificial constructs that hold different meaning for different people, and describe a reality we don't really understand at all - so it doesn't do to be dogmatic about it. If one were to be strict about myalgic encephalomyelitis, perhaps you couldn't really prove you have it until an autopsy reveals inflammation of the spinal cord...
I do think that what seems to be going on with XMRV is the completely unexpected finding that here's a fundamental enabler for neuro-immune abnormalities, and everyone with all of these conditions has got XMRV (or maybe some will turnt out to have something else fulfilling the same functions). So nobody ever expected a single biomarker for any one of these wastebaskets, but then it turns out to everyone's surprise that maybe there is something that is common across all the wastebaskets! It makes perfect sense that XMRV or something like it can unlock the potential for a wide range of other pathogens to cause a wide range of problems, but this "big picture" vision has proved too shocking for people who were focused on particular lines of research.
I think that an awful lot of people in relevant research fields has had - and will have - a hard time understanding some of the implications of XMRV because they seem to contradict the way things were pointing within their own research. But from a more detached and generalist perspective, none of this is too surprising in a multifactorial situation. A massive part of the problem is just the way humans are with words - we give them more reality than they deserve. Whilever we are talking about syndromatic conditions, we just don't know whether what we are describing as one entity really has any meaning at all. When the answers come in, I expect those answers to all run right across all the boundaries that have been established for naming the various conditions. And by the way, think what a logical mockery that makes of, say, looking at a study and saying "oh, only 60% of CFS have this, so it isn't the cause of CFS"...that sort of logical non-sequitor (actually, a series of meaningless statements) seems very typical to me.