Why does Feitan torture when Pakunoda can search someone's memory?
What makes you think that is Feitan's sole or primary purpose?

First, what's wrong with having 2 members of the Troupe excel at extracting information from its enemies? What if one of them is indisposed, but they need to get information out of someone?

Second, he is one of the founding members of the Troupe. While this isn't a huge deal, I doubt anyone would say to him, "hey, by the way, we don't need you anymore. We got Pakunoda now. Goodbye."

Third, Feitan is incredibly strong. He is the 5th strongest member in Phantom Troupe, in terms of sheer strength. He is easily one of the fastest.