[kevkrom]

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.

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.

[Sherman Logan]

data from that source should be treated with caution unless/until it is independently verified.

Good for any source, of course, but degree of caution should increase as the known degree of reliability of the source goes down.