In That Hideous Strength, Andrew MacPhee was an old friend of Dr. Ransom's that appeared as the "resident skeptic" at St. Anne's. Beyond that, he's never shown actually doing anything. A lot of the other characters bring specific expertise (e.g. Ransom's contact with the Oyarsa and role as the Pendragon, Jane's clairvoyant dreams, the Dimbles' expertise in the history of Arthur, etc.) and/or are "outcasts" somehow from the N.I.C.E. (e.g. Ivy Maggs).
Neither of these appear to be the case for MacPhee. I'm not aware of any textual indication of him being an outcast, nor does he appear to have any particular expertise beyond his skepticism. It's not even clear exactly why they keep him around as a gadfly, given that none of the characters ever appear to take his advice or change their opinions in response to his arguments. Whenever he asks for something concrete to actually do Ransom refuses. Ransom also adamantly refused to let MacPhee try to find Merlin.
I don't have the exact quote in front of me, but when MacPhee asks Dr. Ransom exactly what use he was, Ransom gave a rather vague answer that seemed to be more or less to the effect that him just being there was helpful.
Does anyone have clarification as to exactly why MacPhee was there, and how he was actually helpful?