OK I got some information from Liberty Pumps. They said that this pump needs its own discharge to the storm sewer because if the check valve ever failed on the primary pump the city water would flow back into the sump and flood the basement. Makes complete sense.
Running a separate discharge to the outside is quite an undertaking, for me at least. To begin with, it's a tight space in the basement where the pipe exits, I don't have a hole saw that can make it all the way from the exterior through the spray foam in the box sills, and I would have to rebuild all of the existing discharge piping outside.
Would there be any problem with installing a secondary (back-up if you will) check valve on the primary pump's discharge?
This sump pump is meant only as a backup, mainly in the event of a power outage during a storm. As it is right now, in order for my basement to flood from the backup pump there would have to be a power outage or a failure of the primary pump and at the same time, the check valve would also have to fail. Plus, I would also have to be gone as to not notice the endless running water.