How would outing Valerie Plame as a CIA agent "discredit" her husband?

It wasn't the "outing" of her per se, but the implication that his trip to Nigeria was just a "junket" that he got igned because of nepotism. In other words, Valerie pulled some strings to get her hubby igned to this "fun" trip. That story just doesn't work unless you come out and explain just who Valerie Plame is and what her role is in the CIA.