Perhaps those who "reject" a religion's truth-claims should withdraw; perhaps not. I tend to believe there are reasons for many to remain affiliated with a religion (in this case, the LDS Church) even if they can't embrace the truth claims. Those reasons might include things as simple as, "It's the best I know, even if I don't believe it's exclusively true," and as complex as, "My wife (or husband) and kids are happy in it, they embrace it, and I am willing to participate as a non-believing member of record in order to maximize their peace and happiness."
What really concerns me is any exclusion of those who believe truth-claims are "unknowable" - since I believe they actually ARE unknowable to many.
Our own scriptures say that some are given the gift "to know", while others are given the gift "to believe" those who know. I think there is great selfless service that can be provided by those who either believe those who know or even don't believe those who believe or know - if they stay in the Church. I have NO problem whatsoever with members who reject exclusive truth claims retaining their membership, as long as they don't fight the Church. I have no problem with those who think things are unknowable staying the Church - with no disclaimers whatsoever. I think they belong in the Church every bit as much as someone who feels they know everything - or even as someone who actually might know everything.
Fighting the Church is one thing; simply not accepting certain claims is quite another. Simply saying certain things can't be known is at a completely different level. Personally, I would hope that NOBODY whose only "issue" is an inability to accept truth-claims or feel they are knowable would face excommunication over it or be encouraged to leave. I would feel a deep sense of loss and injustice if that were to happen.To say it differently, I don't think accepting all truth-claims (especially the exclusive ones) even is a requirement of worthy temple attendance (or even ward or stake leadership), much less regular membership. I know a highly visible member of leadership in a stake where I have lived who doesn't like the "only true church" phrase because of how it can be and is interpreted by many - inside and outside the Church. I understand his concern, and I share it to a degree. He is a faithful member, with a truly miraculous conversion story, and to think that he should give up his membership simply because he can't accept that particular "truth claim" is abhorrent to me.