What's supposed to happen with ZFS is that if the rest of system needs more memory, ZFS should back off and release some of the memory that it's using. It can do this because most of its memory use is just caching, and while there no doubt is a limit to how far you can take this, I never heard anyone question the effectiveness of this on the OpenSolaris/Illumos implementation.

I don't follow FreeBSD closely, but if memory serves there was a concern, particularly in the early days of the ZFS port, that their implementation couldn't be counted on to release RAM fast enough, if the system suddenly came under significant memory pressure. Hence the advice was to always run with more-than-sufficient RAM to minimize the likelihood of getting into low memory situations. I think this is a significant part of why FreeNAS considers 8GB to be the minimum supported configuration.

So it seems to me that this isn't really about ZFS's RAM requirement, rather it's about hedging against the volatility of the RAM requirements of other software on the same box, in case ZFS can't back off fast enough.

These days I run ZFS on Linux, and I remember about 4 years back spinning up some bulk data processing job that was configured to use 14GB of RAM, on a 16GB box, and watching ZFS's ARC RAM use drop in a single second from 5.5GB to 0.5GB. So I'm satisfied that for my purposes, on ZoL in recent times, this isn't an issue I need to worry about.