If you're running your tub with an SWG and chlorine, MPS is really not necessary. You can simply super-chlorinate with liquid chlorine as-needed. MPS would only be effective in a chlorine hot tub if it is used when the FC is low, for example, after a big bather load. Then you would want to shock first with MPS and allow that to destroy the organic bather waste before adding any additional chlorine. The reason being is that MPS oxidizes bather waste without creating chloramines BUT, if you shock with chlorine first and then add MPS, the chlorine will create chloramines and other disinfection by-products that are harder or impossible for MPS to destroy.
Given that you are using an SWG, you really don't want sulfates in the water as they can destroy the coating on the SWG and, if high enough, will cause accelerated corrosion of the heater. MPS breaks down into sulfates as it oxidizes bather waste.
So, as you can see, it's really not necessary, it's more expensive, and it can cause accelerated wear on critical tub components.
