Skip stop works great in situations where station usage along the skip stop portion is fairly consistent between stops and service is frequent. The headways on the J/Z aren’t great enough to warrant it. Additionally, NYC’s labor market is becoming more fragmented and less centralized in Manhattan, so plenty of people on the J/Z aren’t heading to Manhattan anymore. The assumption that J/Z were going to Manhattan is what that express and skip stop service was designed for. It’s time to axe it.