I think it's safe to say that if your hardware requires data integrity to properly function, and servers pretty much fall squarely into that field, then ECC memory is a
requirement, not just something to be recommended. Servers should never be run on any else, regardless of OS or filesystem used, period. With the exception perhaps, of a quick trial/ test install on a desktop for evaluation purposes etc, there's simply no good reason to use non ECC memory in any server, even if that server hardware may actually be able to boot/ run with it installed. Any higher cost involved for using ECC memory shouldn't even play into any decision to use it or not and if it does, you can't afford to get in the game, it's that simple. You never see or hear of Dell, HP, Cisco et al, or any other server manufacturer, offer you a "cheaper" alternative just because you are not planning on using ZFS, or for that matter,
recommend ECC memory because you plan on using the hardware for a FreeNAS install, that would just be ludicrous. Neither are they just out to make more money by selling you ECC memory, they have a lot of other ways to reach into your pocket, but that is not one of them.
I have seen more arguments, discussions and opinions about whether the cost/ benefits are worth it over the years, than I have had coffee's, and with very, very few exceptions, they are all flawed