You lose a key feature of ZFS, which is scrubbing. This feature requires multiple drives in a zpool, while you only provides one. Scrubbing generally happens once a month to prevent double or triple bit flips in the same location, however you might get bit rot with your service.
You can scrub but not repair, ZFS will tell you of corruption but not be able to repair.
You can also set copies blocks/data (copies=2) which stores two copies on one drive, allowing you to repair. Doesn't cover you for drive failures but can for bitflips. And you'd get half the storage.
