The problem with the down there is that stop is in the middle of everything. Also, its counterparts on 8Av/6Av/Bway all have two locals running at that point during the day. Even though its frequency is high, its fits fewer people per train then its counterparts on the other trunks. The second problem is that unlike its Lex local counterpart where there are heavy stations below GC, the local stations below 34 has light ridership (less real estate to cover being that it has services on either side of it a crosstown block away), and often trains get to 34 white hot if the crew operation is even of average speed. Still, TA will not add extra trains on that line just because of two stations.
I always wondered if it was better to have the express-local setup at 34 be the regular two islands for 7Av service instead of two sides and one island because of this. I understand on 7Av/8Av why its was created this way, but there isn't any kind of problems over on 6Av, which has two island express/local setups three stations in a row.