With apologies for Appeal to Authority: Paul Graham elucidates the low value of Ad Hominem in his post How To Disagree: http://www.paulgraham.com/disagree.html
It's not as though the author's biases or background are completely irrelevant; but discussing them is unhelpful without additional clarifications on the mistakes, ignorances, or dishonesties alleged.
Imagine a different context: maybe I have a strong opinion as a lay citizen on campaign finance issues. It's all well and good for someone to enter saying "I'm a political operative/lobbyist/etc, and you don't know what you're talking about"; but it's a zero-information statement until they describe what they know that I don't (which would be just as helpful and pertinent if I had turned out to be an expert anyway).
It's not as though the author's biases or background are completely irrelevant; but discussing them is unhelpful without additional clarifications on the mistakes, ignorances, or dishonesties alleged.
Imagine a different context: maybe I have a strong opinion as a lay citizen on campaign finance issues. It's all well and good for someone to enter saying "I'm a political operative/lobbyist/etc, and you don't know what you're talking about"; but it's a zero-information statement until they describe what they know that I don't (which would be just as helpful and pertinent if I had turned out to be an expert anyway).