Problem they found back in the 60s was that traffic offering from the north and south stacks was rarely equal - there would be a rush from the north all for 28R (now 27R) and nothing from the south for 28L, causing problems for the #2 Directors (now Final Director). Of course, back in those days there were plenty of staff to provide for two #2s.... but now???
Similar problems occurred with outbounds so although both runways were available the GMC problems were horrific.