I come from philosophy. In that house there's an old and warming idea - that it shouldn't matter who raises a point, because good arguing is nil ad hominem - it has "nothing directed to the person" who's arguing, but all instead to what they argue. This is a noble idea. Unfortunately some new ways of thinking raise fairly fatal problems for it.
- When we talk about Difference in the new way, ad hominem is important. When an argument is politicised (as indeed even the abstractest arguments are), it can matter who is saying what. Some philosophical topics draw on experiences which are not universal nor easily mentally simulated.
For instance: gender. It's not hard to see what's problematic about a man stomping around telling a group of women what feminism should be, no matter how sympathetic and well-informed the man is, or how hostile to feminism the women are. Part of male privilege is to be disproportionately valued in discourse. Part of feminism for the foreseeable future will be to make men qua men uncomfortable. I can handle that.
- And, from a very different skew (which funnily enough bends right round to meet up with the first point): let's talk Nietzsche. He and other supporters of "psychologism"-about-philosophy feel free to evaluate philosophies based on their philosopher rather than their truth. Questioning the motives and complexes of thinkers is valid for them because beliefs and arguments are embodied and situational things, always made by some flawed person for a reason. It's an anti-foundationalist thing, stupid. It's also great fun.
The nagging-child question "Well, why do you think that?" is invariably eventually plugged with an attitude, and not a "proper" justification. "Because I am inclined to it" is exactly what their ad hominem aspersions aim to get you to admit.