I’m not sure if you rode the back in the 80s and 90s (when it was running via Montague), but I did. During weekdays, when they couldn’t send all the trains to Sea Beach, they short turn trains at Canal (not Whitehall) and it worked like a charm.
I ride from QBP to DeKalb Tues-Sat, and most of the time, I see the empty at Prince St southbound during the peak of the AM Rush. Astoria needs the extra service, and you can’t send more trains through DeKalb, that’s the only reason the supplemental service exists.