It is correct that part of the changes after making Syracuse an interchange was all interchange was done there, and they did make some kind of haulage agreement after the line went out of service. The last trip the F45 made was on one of those trains, I happened to see it one night at a crossing in Oneida. What is current on that, no idea.
I want to say that after the move to Syracuse, Conrail made some changes in Utica including eliminating a remote interlocking and the connection to the NYS&W became a hand throw.
But my thought is being able to go to Binghamton opens up interchange for the traffic to go to NS. Even if it can still go there when going haulage rights to Syracuse, I would think it would save at least a day and some expense. And it makes NYS&W look better to run even an occasional train on the line.