What a lot of people don't realize is that it's not only helpful in power outages. Even if you have a home standby generator to keep your AC power up and running, your primary pump could still wear out and fail on you.
Regardless whether you have power running to your sump pump or not, if your pump fails and you don't have a backup, you don't have a pump.
You may be thinking, "I'll just get another primary pump and plug it into the AC," but that's not a good idea either. It's recommended that the outlet you plug your primary pump into have nothing else plugged into it.
For this reason, you should still opt for a battery backup sump pump. Also consider installing an alarm to sound when it kicks on. This way, unless the primary pump is simply having trouble keeping up, you'll know that it's time for a replacement.
Moral of the story, just because you have an emergency generator doesn't mean that you shouldn't have a battery backup sump pump. They're crucial in keeping your basement above water whether there's a power outage or not.