Hi Julia,
It is implied in the earlier part of this story that Barnabas is the only one who can really handle Adam, emotionally at least.
At this point, though, Nicholas fancies himself in complete control of da big lug, so I have not a clue why Barnabas' presence is considered essential at this stage of the game ... apart from the fact that in 1968 the ratings for individual episodes were supposed to be higher whenever he appeared!
Gothick