If you ask me, Desmond did a much better job [spoiler]defending Quentin during his witchcraft trial in 1840 than Peter did Vicki at her witchcraft trial in 1795. Desmond successfully was able to get Samantha's clearly spiteful and prejudicial testimony against Quentin thrown out, while Peter couldn't even put a dent in the damaging, perjuring testimony of Nathan Forbes.
Had Desmond not been wrongly implicated in practicing witchcraft himself, I think he could have gotten Quentin acquitted.[/spoiler]
If I were ever in trouble I know I'd want Desmond to represent me. He was far more experienced at practicing law than Peter was.