Really? I use it often enough, and it always seems relatively busy. It's the terminus for the Parliament bus (and the Wellesley bus, though I confess I haven't used it east of Yonge), it's next to a large high school, and it's the nearest subway station to the Brickworks.
Looking at the ridership numbers, it's about the same as Museum. Should we close that one too? Quite frankly, looking at the ridership for various stations, only Bessarion sticks out as being low.
Castle Frank is about 8,000 riders a year. Glencairn and Summerhill are about 6,000, despite much in the way of buses. Rosedale is 7,000.
Most cities would be happy to have 6,000 riders per station. Looking in Vancouver,
a lot of Skytrain stations are less than this, even in areas where they are quite far apart (add the 2 columns together to compare to TTC numbers).