It is appropriate mainly when the person being attacked is speaking from a position of authority. IOW, he making the argument that we should believe what he says because HE is saying it.
One of the most common uses of this fallacy I see on FR is those who discard any evidence provided from a source they don’t like, such as Wikipedia or Snopes.
I agree that such sources should not be taken as Holy Writ, but they often provide good information when used with care.
If you have proof an argument is invalid, present that proof. That evidence you dislike is provided from an arguably biased source is not adequate reason to completely discard that evidence.
Indeed. I enjoy the people who harbor nothing but scorn and contempt for the MSM...
...until they publish an article supportive of their values, arguments or beliefs.
That's a separate fallacy called "appeal to authority".
One of the most common uses of this fallacy I see on FR is those who discard any evidence provided from a source they don’t like, such as Wikipedia or Snopes.
This one is "appeal to motive", slightly more specific than "ad hominem". Appeal to motive, while a fallacy, can still be useful because it notes that the source can be considered suspect, therefore data from that source should be treated with caution unless/until it is independently verified.
"Appeals to authority" is a logical fallacy of it's own, and pointing out that fallacy in an argument is is not ad hominem, since it is directed at the argument itself, rather than the person.
“Poisoning the Well” (Person “A” lies, therefore you can’t ever believe anything he says) is another logical fallacy popular with liberals, along with the Argument from Authority and the Appeal to Ridicule.
“One of the most common uses of this fallacy I see on FR is those who discard any evidence provided from a source they don’t like, such as Wikipedia or Snopes.”
Bingo! This is one of my pet peeves about posters here and on many other blogs.
Take "evidence" from either with a grain of salt. That's just good judgment.