In this case, the pump is the backup. My business is coming up on 12 years in this building. It actually has very good passive drainage and didn't even have a pump until we ended up with some water in there during Sandy in 2012. But with roughly 6" of rain in a few hours yesterday, the little creek behind the building got high enough that the anti-backup valve in the passive system closed, leaving the pump as the only way to drain things. Shortly after the rain stopped and the water in the creek receded, it started to drain without the pump. By the end of the day, we only had a few puddles left on the floor.
I think the biggest problem with sump pumps is that in the kind of weather where you're likely to need one the most, there's a decent chance the power will go out as well. That wasn't our problem yesterday, but it's quite common, and then even a backup pump won't help.