One of the common copouts used by Witnesses when they are confronted with the real truth about the Watchtower is this:
"Oh you've just been talking to apostates and disfellowshipped ones, or you've been reading their literature. They can't be trusted."
When that comes up, ask them, "Do you believe that Jim Jones, the leader of Peoples Temple (mass suicide cult) was inspired of God?"
They will be quick to say, "Of course not."
At this point, you can explain to them that EVERYTHING we know about the inner-workings of Peoples Temple and the evil that went on, comes from ex-members. The real truth about Jim Jones didn't get published or circulated until the media started talking with ex-members of the cult. If not for that outreach, or for them speaking out, they would have probably carried on for much longer. (It was the pressure from media and the authorities that eventually drove Jones to move the cult to South America.)
So to say that NOTHING an ex-JW says is to be trusted would be the same as calling these ex-members of Peoples Temple liars as well, and leaving the possiblity open for Jim Jones to be an inspired prophet of God, as he claimed to be.
Hope this made sense.