I think ad hominems can also help establish trustworthiness.
Recently, a guy mostly unknown to the general public put a ballot measure for a new city ordinance on the Denver November ballot. It called for the creation of a new commission, specifying the qualifications of the appointed members.
Those who have followed this person's antics in the past did not trust him, and could identify slightly ambiguous parts of the ballot language, and member qualifications, as being tailor-made for his questionable friends. They could also find things that he said would be done or would be true if people voted for the initiative, to be false and unlikely (as in the case of some political candidates). And things that MIGHT be good or bad, depending on the person's true intents, they felt would be bad, considering his track record.
And so, one of the tactics they used to oppose the ballot initiative, was to use ad hominem attacks: bringing to light dumb things he's said and done in the past; pointing out his lack of education and qualifications for constructing a ballot initiative; attacking the topic of the initiative, rather than the initiative itself; listing the questionable motives of his friends; and so on.
This was a minor part of the opposition campaign. But it seemed worth while, since the ballot initiative was a new thing for many people, and so was the author. Deciding whether it was a good idea or not went beyond just what was written down: it also included the trustworthiness of the author, and what he and his cronies has planned were this to actually pass.
So I think in such a case, to help establish the trust of the person, ad hominems can be a useful tool. Not the only tool, and probably not the primary tool. But it can help shed some light on important areas otherwise in shadows.