I personally don’t like talking about baptisms for the dead in terms of “becoming members of the Church”. Baptism in this life leads to membership in a church; I just don’t see it as having the same connotations in the hereafter. When someone accepts a proxy baptism, I don’t see them as joining a church; I see them as giving evidence of their acceptance of their Savior and His redemption.
Perhaps it's just me, but I believe churches are mortal organizations. I just don't see "churches" existing in Heaven.