In addition, I'd like readers to consider this scenario: the goes up 2 Avenue and ends up skipping 49 Street full time. This station which should have been an express station (given how the MTA handles Broadway currently) loses a third or more of the trains passing through it. On average, a third of the riders would need to make a transfer and/or walk from another station. This single station is used by over 8 million riders annually (and rising). To affect nearly the same amount of people in an outer borough like Brooklyn, you'd need to do something like make 2 out of every 3 trains run express between 9 Avenue and Bay Parkway.
My point: express service is a disservice to riders getting on and off in the central business district, but a useful service outside of there provided there is a ridership to support it.