I'm not convinced that 'Christianity' is a single thing, or that there's really such a thing as 'true' Christians. In my view Christianity is a diverse and heterogeneous family of historical tradition, defined by having its origin and by finding its inspiration in the person of Jesus.
We can identify several different varieties of very early Christians, such as the Jewish Christians of Jerusalem, centered around Jesus' brother James, the churches established by Paul's evangelical mission to what is now Turkey, the so-called 'gnostics' that became popular in places like Egypt, and so on. It might be kind of an historical accident that the Jewish Christians were wiped out in the Jewish wars of 70 and 130 CE, and the gnostics divided up into lots of little mystical sects and fizzled out several hundred years later. So it happened to be Paul's community up in Turkey that collected the most important of its own in-house writings in the form of the 'New Testament', which has come to represent what most people today think of as normative Christianity.
Certainly the Mormons adding new inspired writings alongside what mainstream Christianity had already come to think of as a fixed Biblical canon does separate them from mainstream Christianity.
We can look at historical parallels of that. The early Pauline Christians' addition of their 'New Testament' to the existing Hebrew scriptures was never accepted by mainstream Jews. And during and after the Jewish wars, when the Jewish Temple was destroyed by the Romans and the Jews were driven out of Jerusalem, passions grew so hot that traditional Jews refused to accept that Christians really were Jews at all, not even heretical ones, while the Christians insisted that they enjoyed a brand new covenant with God, through Jesus' incarnation, death and triumphant rise, and that they were in fact God's new chosen people. So the two traditions split like a Y. The mainstream Jews, originally far more numerous than what was originally a small Jesus cult, retreated into themselves after losing the Jewish wars and became a closed hereditary caste. Simultaneously, the Pauline Christian side of the Y became the Hari Krishnas of their day, busily evangelizing their 'gospel', trying to convert everyone who would listen, and eventually became vastly more numerous and powerful than the old and shrunken Jewish community.
Contrast that with Buddhism. The Buddha lived in the 400's BCE, and his early monastic community evolved and elaborated for several hundred years. Around the same time as Jesus, the mainstream Buddhists established a fixed canon of writings (actually several variants on it, native to different schools) embodying the tradition of the early centuries. The Pali canon is an example of this. These writings represented the traditional doctrine of conservative Buddhism, the Buddhism that tried to remain true to the earliest teachings.
And alongside this, more avant-garde monks were continuing to write a whole host of new sutras, containing new ideas and new doctrines. These came to be called (by their proponents) Mahayana sutras. Interestingly, the vastly enlarged Mahayana canon never really closed. New writings continued to be added to it for many centuries, and today nobody really knows precisely how many Mahayana sutras there are.
So Buddhist history has kind of a Y shape as well. But unlike the Jewish-Christian split, the division in Buddhism didn't result in two self-consciously different religions. Buddhists on both sides of the Y continued to think of their opposite numbers as Buddhists. Of course the conservative Theravada sometimes thought of the Mahayana as espousing fantastic and heretical ideas, while the Mahayanists sometimes thought of the conservatives as following a crude and inferior form of Buddhism suitable only for lesser aspirants. I guess that the Theravada/Mahayana division in Buddhism might be more akin to the Catholic/Protestant division in Christianity. While both sides of that split criticize each other, they still acknowledge the other side as 'Christians'.
So it's interesting to speculate on which way the LDS/traditional Christian split is going to go in coming centuries. Will the Mormons gradually deviate to the point where they and other Christians no longer recognize themselves as belonging to the same religion? The Jews and the early Christians evolved that way. Or will the LDS/traditional Christian split be more like the Theravada/Mahayana split in Buddhism, expanding the scope of what the word 'Christianity' contains and adding new scriptures to it, without totally rupturing it?
It's religious history in the making, as we speak.
