The main reason to have a ws fitted is to make the ro membrane last longer.
If you live in a soft water area then don't worry about it. It's probably not worth the worry unless you have an expensive ro system either, my softener is a 3500L one and set me back £400ish you can buy them smaller and cheaper but if your using an ro man or something similar the ro would be cheaper than the ws so why bother. Very good for 4040 systems or merlin though.
The ws goes after the mains water before the ro.
Simon.