In my humble opinion, softeners are pointless, even if they extend the life of your membrane.
Here's why:
Say you spend an average 5 minutes each working day faffing around with your softener. Either backwashing it with salt or measuring the output or flushing it out to make sure all the salt is gone or whatever. 5 minutes is not much time, but time is money.
If you work 4 days per week, that's 86 minutes a month, or just over 17 hours per year.
17 hours @ £25 per hour is £425. I know most (or all) people on here will earn much more then £25 per hour, but being conservative
A brand new 4040 membrane costs £323 incl VAT and shipping and including an hour of your time changing them over.
So even if you have to buy a brand new one every single year it will still be cheaper than faffing with about with a softener. And that's before you even pay for the softner consumables such as salt.
Just my opinon.