It is true that hard feedwater can lead to the premature demise of an RO membrane. The dissolved minerals (primarily Ca and Mg) in hard water form scale that most people are familiar with. The scale can form inside the RO membrane. When you "scale" a membrane, purified water production slows, and often the quality of the RO water declines.
In commercial applications where 1) the RO membranes are larger and can get quite expensive; and 2) membranes are commonly plumbed in series (concentrate from membrane 1 feeds membrane 2); and 3) less then high performance of the RO system adversely affects commercial operations; we routinely specify water softeners. Where the RO system is expected to be online at all hours of the day, we typically specify twin tank alternating softeners - these softeners have two mineral tanks (tanks containing water softening ion exchange resin) that can produce soft water 24/7 without interruption.
That said... the residential-scale RO systems used by homebrewers use relatively inexpensive membranes (~$30 to $50), are rarely configured with membranes in series, and are typically configured with a lower recovery (more of the feedwater ends up as concentrate) than commercial systems. The user with hard water is essentially making a trade off - he's accepting lower recovery and or shorter membrane life (higher operating expense) to avoid the expense/trouble of using softened water.
The choice is up to the user of course. If the only consideration was the welfare of the RO membrane and the RO system... it is a no brainer: always use soft feedwater.
Russ