My wife comes with me to mine, though if I ask her to sit one out she's fine with that. I think it's a safer place to discuss hard topics with her when there's a third not related party there to guide the conversation if need be. My therapist has done that with us before and it really helped the situation and our communication. Like Fharraige said, she too offers input and can give the therapist an outside perspective of situations that the therapist can help me with.

Best of luck with your therapy and your wife.