Poisoning the Well, or Pointing Out that the Well is Poisoned?

So I've been thinking (yes, I know, a very dangerous exercise) about certain fallacies we all learned in our Logic classes, particularly the ad hominem/ poisoning-the-well fallacy. And of course, we have been taught that one ought never to use this argument because it is fallacious and invalid.

Here is an example of an ad hominem argument:

"We have all witnessed Steve's proclivity to misrepresent people and their views, and have heard him make wildly false claims about everything under the sun. Therefore, we should not believe anything he says about Chinese food" (See post below).

Obviously, this example shows why we shouldn't commit this fallacy. But on the other hand, isn't the story about The Boy Who Cried Wolf there to teach us that there is validity in the ad hominem? In other words, the moral of the story is: liars are not to be trusted, so don't be a liar. Or maybe it's just the other way around: You should probably trust a liar, because sometimes he is telling the truth (the boy was telling the truth the last time). Now, I'm not excusing all ad hominem arguments by any means; I'm just asking if they do indeed carry some weight sometimes. Could it be that sometimes we are not poisoning the well, rather we are simply pointing out that the well is already poisoned? For example:

"I was reading a book by Vijay, and in it, he made claims regarding the New Testament which I know to be false (since I am an expert on the New Testament). Therefore, I don't know if I should trust the things he said about the Old Testament either (since I'm not an expert on the Old Testament). If I know he is wrong about the subjects with which I am familiar, why in the world should I trust him regarding the subjects with which I am not familiar?"

Anyway, I'm not trying to come to any conclusions in this post; I just want to start some fun discussion. So, discuss!