First of all, who said that trials must abide by the rules of logic? The goal of a lawyer is to convince a group of the 12 most pliable people they could find by any legal means necessary. They often make use of fallacious appeals to emotion, ad hominem attacks, and other such nonsense. It works very well because most people are not logical by nature and those who are tend to get weeded out of the jury pool early on by whichever side is wanting to make the emotional appeals. It's typical in trials for one or both sides to make emotional appeals that are completely fallacious.

Having said that, there are non-fallacious ad hominem arguments just like there are non-fallacious appeals to authority. Judging the character of a witness as a means of determining the credibility of their testimony is one such case. It isn't fallacious because you are not judging a conclusion that a witness came to, you are judging whether they are being truthful about the experience they claim they had. When you start doing that with the expert witnesses (who are really testifying about conclusions they came to based on evidence), then it can start getting into fallacious territory when lawyers take that route (and they do).