Disclaimer: This may or may not directly answer your question.
ZFS is an advanced block storage technology, primarily geared towards enterprise-level storage arrays.
Let's take a look at man zpool: (interesting parts are highlighted)
Virtual Devices (vdevs)
A "virtual device" describes a single device or a collection of
devices organized according to certain performance and fault
characteristics. The following virtual devices are supported:
disk
- A block device, typically located under /dev/dsk. ZFS can use individual slices or partitions, though the recommended mode of
operation is to use whole disks. A disk can be specified by a full
path, or it can be a shorthand name (the relative portion of the path
under "/dev/dsk"). A whole disk can be specified by omitting the slice
or partition designation. For example, "c0t0d0" is equivalent to
"/dev/dsk/c0t0d0s2". When given a whole disk, ZFS automatically labels
the disk, if necessary.
file
- A regular file. The use of files as a backing store is strongly discouraged. It is designed primarily for experimental purposes, as
the fault tolerance of a file is only as good as the file system of
which it is a part. A file must be specified by a full path.
This clearly states that the intended mode of operation for ZFS is to use whole disks as virtual devices (vdevs), which is also the only thing that truly makes sense in a production environment.
The use of single partitions as a vdev can maybe be useful for home-users in extreme edge cases. And the use of files as a vdev can only have one meaningful purpose: Testing.
The entire performance and reliability of ZFS is built around integrity and redundancy - and you loose both those things if you can't easily replace a vdev - and this is only possible when you can swap an entire disk.
There are plenty of testing and experimental usecases where it can be handy to use partitions and even files as vdevs - but as soon as your setup enters any serious mode of operation, always use entire disks as block devices.
Edit: For consumer use with a single harddisk, it doesn't make any difference if you use ZFS on a partition or the whole disk. Here, you only gain the filesystem advantages, such as snapshots, integrity checking etc.
