This is a dedicated NAS. What workload do you expect to be paging to disk? Your primary workload is already reading and writing bits off disk.

Because we live in the real world and no system is truly in a bubble if it's connected to a network. But mostly because seeing oom-killer messages in dmesg makes me sad :).

If something gets leaky enough then it's still going to hurt performance for a while and die.

Swap delays that but extra RAM delays it too. If you take a use case where 2GB of memory is fine, and give it 8GB, then you already solved the problem swap would solve. You can always add more but you're past the point of diminishing returns.

Since there's already a non-ZFS system disk, it's a little weird to not put a swap partition on it.

But I wouldn't say it's questionable. On a NAS I doubt you'd ever use even one full gigabyte of swap space. Keeping it simple is fine.