mtbarr64,
Free Chlorine (FC) at the proper levels relative to Cyanuric Acid (CYA) will kill bacteria and inactivate viruses much faster than either silver or copper metal ions. It also inhibits algae growth though copper does a good job with that as well. The problem in a spa is that many people use Dichlor only and the CYA level builds up to high levels making the chlorine much less effective. This can allow for more resistant bacteria such as the one that causes hot tub itch to grow. Also, one must be more diligent in a spa since the chlorine gets used up more quickly due to the higher bather load, higher temperatures, and greater aeration. Also, most people don't use enough chlorine in their spas so it gets to zero before the next soak.
So basically, Nature2 can be seen as insurance against letting the chlorine level get too low. It is analogous to someone using PolyQuat 60 in their pool to prevent algae in case they let the chlorine get too low. Nature2 is not necessary, but it isn't ineffective -- it's just not as effective as chlorine alone. Nature2 with MPS is better than Nature2 alone, but only in the higher temperature of a spa, though it's still not as fast a killer of pathogens as chlorine alone. The metal ions, especially with MPS, kill fast enough to prevent uncontrolled bacteria growth, but not fast enough to prevent person-to-person transmission which is why you won't see it used without a fast acting sanitizer (e.g. chlorine, bromine, biguanide) in a commercial/public spa (or pool).
Richard