This is a response I got from a leading expert in water treatment:-
"One can use an RO system which also functions the same way as MB resin; i.e. it removes both cations and Anions. If the cationic load is very high, we can supplement this system with a water softener prior to the RO. The softener is a Cation exchanger and removes only the Cations from the water, thus reducing the load on RO. This way it does optimize the performance of RO, but adds on two systems for operations; i.e. Water softener and RO.
Frankly when one can use only the MB, I don’t see any use of water softener followed by the RO. Some of the benefits of RO pertain to making the water potable etc are not relevant for window cleaning. "
In summary if your Cation load is high yes a water softener will have an impact. If it is low it is a waste of money. MB is a 60/40 mix so if cation load was high then your cation mix would be the first to be exhausted so makes sense, if low your anion resin would be first to exhaust so makes no sense to put a softener prior to RO as you are throwing money away.
Hope this makes sense.
Cheers
Doug