Note it won't necessarily halve the capacity. Selectively enable it for the datasets requiring it, and avoid the overhead with the rest.
No but parity archives solves a different problem, with only some percent of wasted storage you can survive bit-errors in your dataset. It's like reed-solomon for files.
In order to achive the same with ZFS you have to run RAID-Z2 on sparse files.
Anyway I will try to use it for my main PC which has several disks and continue to use my solution for single disk machines (laptop, vps, SoC...). :)