Landy, why did you just bother to make that comment? It's obvious that some JWs do wake up.
You're correct, they do. But it doesn't happen by someone being a biblical smartarse at a door they happen to be knocking on. As Nic says, it tends to reinforce their beliefs.
When people leave the witnesses they tend to do it through self realisation, or more commonly, through apathy. It's only once they've stepped away from it they can see it for what it is.
A website like this will do a lot more to help people see the religion for what it is than standing at the door preaching at them. In this particular case the op just seems to be other side of the JW coin.
The other factor is that people who engage in this sort of confrontation often do it to service their own ego rather than it beng a genuine effort to help someone else.