It's a weak backup, but sure if you want to use it that way you can. The best is to maintain an FC level at all times, but it's true that if it drops too low or to zero that if there's silver ions in the spa then that should inhibit bacterial growth. Of course, you could just see about using MPS instead since it won't deplete as quickly as chlorine and that would provide greater disinfection capability so be less risky of things going south. It's easy to switch back and forth between using MPS vs. chlorine or even having some of both at the same time. It's just that MPS will measure as chlorine in the tests, but with Nature2 it doesn't really matter since both act as a disinfectant (at least at hot spa temperatures, for MPS; chlorine does well even in colder temperatures).