I heard that line of crap many times myself. But, what they don't tell you is that what they know is all based on a translation of the Bible that is deliberately tampered with. What they know is all encapsulated into the theme of the Watchtower doctrine, which is what they tampered with the Bible with a view to highlight.
The truth is that most theology students know more about the Bible itself than any Witless. They know it because they study the actual sources, and in context. This is harder than taking pieces out of context to support some stupid doctrines. And so it is going to take longer for them to understand what the Bible actually says. In fact, there is so much heated debate about how the Bible should be taken--whether the missing books are actually part of the Bible, whether the Bible should be taken literally or as a parable, and if it's even valid. That shows me that the Bible is an extremely difficult work to understand correctly, and one viewpoint is as good as any other (as long as it isn't deliberately tampered with to support some nefarious purpose).
I feel that I know more about the Bible now as an apostate than I ever did as a Witless. I know that the Witlesses take pieces out of context to support their nefarious aim, and all I have to do is find the missing pieces to better understand it. I understand that Christmas and birthdays are actually scripturally allowed (Luke 2:10-14 supports this). I understand Ephesians 2:8 and 9 as directly clashing with the Watchtower Society. And I understand that the dashes found throughout the New World Translation are there because the text would have undermined the doctrine of the Puketower Society.