I have never heard a satisfactory answer to the question of what is wrong with exclusiveness per se, and yet it is a common enough — one might say, thoughtless enough — assumption nowadays that there is something wrong with it. The answer usually comes as a restatement of the assumption: “Well, it excludes people, and that’s bad”. The ostensible concern, I presume, is that no one should be excluded from society, or some part thereof, if he does not wish to be ; but that does not explain the antipathy against exclusiveness per se. This antipathy is a curious phenomenon, and a destructive one too, as Richard Weaver noted:
The questioning of apartness, the suspicion of difference, the distrust of distinction, the jealousy about allowing privacy—these are all features of a modern mentality which, often without even knowing what it is doing, may put an end to what has always been the source of culture — a particular kind of development in response to particular values. Thus the plight of the individual is re-enacted on a larger scale. Not only is the single human individual being pushed toward conformity, but the individual group or culture is met with the same demand to go along, to become more like the generality, and so give up character. 
Perhaps once again we see the insatiable nature of power, which lusts for the inclusion of everything, such that an ostensible concern for the inclusion of everyone can become the insistence that no one may set himself apart.
 The belief that no one should be unwillingly excluded from society, or some part thereof, has its own problems.
 R.M. Weaver, “Reflections of Modernity”, Speeches of the Year, Pamphlet, (Provo: Brigham Young University, 1961), reprinted in In Defense of Tradition: Collected Shorter Writings of Richard M. Weaver, 1929-1963, ed. by T.J. Smith III (Indianapolis: Liberty Fund, 2000), p.113.