Part of the reason the whole "corruptions of Scriptures" argument doesn't always work is that for the "corruption" to mean anything, the Scriptures have to be "the" ultimate form of revelation and religious enlightenment for those who use them.
A common mistake we often make upon leaving the Jehovah's Witnesses is never to update our understanding of what those in the religious world outside of the Watchtower actually believe. The idea that the Scriptures have been shaped by outside "corrupting" influences is not news to the majority of those who use them religiously. It only causes a problem for those who have a mindset like the Jehovah's Witnesses.
Jehovah's Witnesses view the Scriptures as the "ultimate authority" on religion and theology. For them everything has to be based on what the Bible says. But for Christians like Catholics, the Orthodox, and most of mainstream Protestantism, the Bible takes second place to Jesus, the apostolic college, and in many cases the teaching experience of the Church body as a whole.
And as for Judaism, the Scriptures are a product of their religion. The religion of the Hebrews functioned for millennia without the Jewish Bible, and in fact it was not until the end of the Babylonian exile that any of its books became finalized into the forms we now have them in. It would not be until the time of the Masoretes (between the 6th and 8th century CE) that anything resembling a formal "canon" of Jewish texts would be settled.
But for Jehovah's Witnesses, the Bible is like the Book of Mormon is to Latter-day Saints. They treat it as if it was the only link to "salvific knowledge" from Heaven. As such it is taught to be the ultimate source of revelation from God, a source greater than the Christian Church that wrote it, the apostolic college that taught its truths before these were placed on parchment, even a greater witness of God than Jesus Christ (for Christians). Unless it is written in its pages in a translation approved by the Witnesses, there is no greater source of authority and truth anywhere else.
So for them the Bible has to be without interpolation from outside sources. When a person who has been a Witness discovers this for the first time they often mistakenly project this "new discovery" upon all religious people who use Scripture, thinking that all other religious traditions view the Bible as the ultimate form of revelation and basis for all "true" religion.
The opposite is true. Jews have known since they developed the Scriptures that many of the ideas they assembled into it required updating. The "Document Theory" (adopted by most in Judaism and Christianity) suggests that the Torah or Pentateuch was written by a series of writers over many centuries. This methodological analysis identifies countless areas where one can easyily see redactions made as the books came into final form over different eras. There are anachronisms in much of the redactions that some interpolations were added to some sections, such as in much of the "P" or priestly material which suggests redaction during the time of the Temple. The Document Theory is totally rejected by the Jehovah's Witnesses because it makes their view of the Bible impossible.
In the end, it is like Half Banana explains: the religious world outside the Watchtower is very much aware that the Bible evolved over time to become the book they canonized. When a religion like, Catholicism for instance, says that the Bible was "inspired," they include the redaction or editorial/interpolation process by various writers and scribes as part of the inspiration process. Since unlike the Witness, Jews and Christians see the Bible as only the "written" part of revelation from God, it's words don't have to be exact truth or pure since the written part of revelation is a product of the experience of revelation that, in most cases, was also an evolutionary process. One doesn't have to accept the Bible as it is, but one shouldn't reject the Bible just on the misrepresentation made by groups like the Jehovah's Witnesses. They aren't right about most things, but that's no reason to dismiss most things.
