It's an interesting position brought out by the question. I think that there are a couple of elements to establish. The first is that I am not sure that Miller is contradicting the idea of belonging. Certainly, characters like Francis and Rebecca Nurse, Giles Corey, and Proctor all "belong" to a certain context, as does Abigail and her gaggle of girls. The idea of "belonging" is still present. Even Proctor "belongs" at the end to a community where transcendence and idealism reigns. I am not sure that Miller is saying that "belonging is impossible." I think that he is stressing that there are limitations to one's desire to belong and that this should not trade off with our own sense of dignity and honor. When Mary Warren is incapable of being honest in court because she fears being left out or "not belonging," it is one of Miller's statements as to how human beings in times of crisis must try to rise above what they should not be and aspire to what can be. Miller brings this notion out in the accusations themselves, started and made against people who "did not belong" to the elite or the upper crest of society. Tituba was no Putnam, and thus it was easy to make her a target. Miller might be suggesting that while belonging is important, it should not be done at the cost of others or at the sacrifice of our own dignity. It is not a contradiction of the concept, as much as I see it as a clarification or type of qualification for it.