Intactivists often get accused of making ad hominem attacks against those who support male circumcision. Outside of attacks on the circumfetishist community, I haven't seen much of this behavior myself, but I think the issue is worth discussing.
In a formal academic fact-based discussion among reasonable people, attacking a person's character, behavior, or actions is quite irrelevant to any argument or discussion. Outside of a purely academic discussion, however, the parameters are different.
Character, trustworthiness, and motives matter. Would you trust an oil company's "study" on damage from an oil spill? Would you trust a psychologist who was also a pedophile to study affects of child abuse? Of course not. Individuals with a vested interest in a certain point of view are not going to uphold the values of a reasoned discussion. These individuals use irrational statements, straw man arguments, or make unfounded claims to divert attention from an issue they can't defend. A purely academic discussion is impossible with these individuals.
To call into question the motives, bias, or vested interest of these individuals is not a "personal attack": it is stating a fact that gives perspective and insight into the discussion. The ideal is not always possible. It is rarely productive to discuss an issue with someone whose vested interest would prevent him from ever approaching an issue with an open mind.
When Ad Hominem is Appropriate
When an individual makes it clear he has no interest in a proper discussion of an issue and his biases are based in unethical, immoral, or evil associations or actions, ad hominem attacks are absolutely appropriate. An individual's character is based not just on his own actions, but his associations. Associating with others who are evil or immoral reflects on an individual's ability to have an open mind. Furthermore, those who lack morals or judgement have very little credibility because they lack the ability to properly analyze and consider all sides of an argument before drawing a conclusion. These individuals have no interest in a proper academic discussion and they serve only to further their cause. There is no shame in revealing the true nature of unethical or immoral individuals.