Jason summed it up nicely. The SWG for spas makes most sense for those who don't soak every day or two since it's a way of maintaining a background level of chlorine in the spa. This is especially necessary when one has an ozonator since ozone depletes chlorine. When one does soak in the spa then one would usually need to add additional chlorine after their soak to handle the bather waste. If you have an ozonator and if you don't soak for very long, say one person for 15 minutes, then you can probably get away with just having the ozonator and SWG take care of things. The only way to know for sure is to test the chlorine level, say, 4 hours after the soak to make sure you still measure at least a small residual of chlorine (i.e. not < 0.5 ppm FC) and measure again around 24 hours after the soak to see if the level is back to the expected amount (usually 1-2 ppm FC if that's where you like to start your soak).
Also note that some spas do not use high quality stainless steel so are more subject to metal corrosion from higher salt levels. It may also void the warranty for spas still under warranty.