Simon,
I asked my mother this very question a little over ten years ago.
My mother's response: Even if they (the WBTS) are totally wrong, totally off the track, I love Jehovah, I've dedicated my life to serving Jehovah, and preaching His message as a witness of Jehovah is the best way I know how.
Me: Could you or anyone else come to understand God's message without the Watchtower?
Mom: (Thinking, a bit) Yes, because all scripture is inspired and good for setting things straight.
Me: So if the Watchtower is off the track, you could just preach the Bible, right Mom?
Mom: But it's the Watchtower that brought me and so many others to this understanding...
...And on and on it went, circularly.
Of course, then I had some improperly made quotes the WBTS had made in it's "Evolution book" and the 1975 fiasco to highlight as evidence of being "off the track" and now there is much more powerful stuff that has been brought to light. But I won't be bothering my parents with this new info. All I will get for my trouble is to be moved from the "sad, faithless son/where did we go wrong" category to the "evil, apostate, devil controlled son/avoid at all cost" category. I've already spent time in the latter category and it took years and some constructive forgetfulness on the part of both parties to go back to the former.
Now, let's see...how can I be of more help to the Silentlambs cause?
Eric