I use them as backups too. At first I turned them on along with my regular lights. But on a long ride, the batteries died in my main front light. I had to rely on the BikeArlington light up in front.
Now I leave the BA lights off so that the batteries are still charged for when I really need them. Maybe I can keep them in working condition until next winter when I pick up more freebies! (Or I could get some small backup lights on my own.)