“It was Epicurus who said that,” you object, “What business have you with what belongs to somebody else?” Whatever is true is mine. I will persist in deluging you with Epicurus, so that people who swear fealty to a school and regard not the value but the source of dicta may realize that the best things are common property.—Seneca, Letter No. 12 (“On Old Age”)
Seneca has no hesitation in quoting a philosophical rival. His advice to his friend Lucillius, about truth being common property, makes the point that while truth is unitary, in everyday life it tends to show up in patches and fragments. Those who teach good doctrine can obscure it by their natural short-comings, while people espousing bad ideas may still offer useful lessons. Thus even the hedonist Epicurus offers advice that the pious and virtuous Stoic can emulate: “The beginning of salvation is the knowledge of sin.” Such a statement is rendered all the more powerful for coming from such an unlikely source.
That said, there is need for prudence. The yield of wisdom must not be outweighed by the dross surrounding it. I might quote something sensible by Karl Marx (if I could find it) if only to make a compelling point against Communism. But I would not do so in a way that lends him undeserved credibility. In heated debates some people become contrarian and look for intellectual allies at the opposite extreme. The danger is that they appear to espouse a position that is not their own or (just as bad) they offer so many qualifications that they become confused and indecisive. To summarize: a positive reference to philosophical opponents has to fit the proper context and be convincing.