That recommendation is slightly outdated. Time was partitioning a disk to use in a ZFS pool interfered with with the ARC mechanism, and that was the only reason for not creating partitions. That's no longer the case, so the major downside of partitioning a disk is gone. (And good thing, too, or root-on-ZFS would be impossible).
Now I can't recall where, but I read somewhere that without that major downside, it's actually preferable to partition the disks. Doing so guarantees that all data on every disk in a pool is physically aligned on the disks properly. If someone knows better than I do, then I'd be glad to be corrected.
SUPER LATE EDIT: To be clear about my second point there, the idea is that aligned partitions matter when mixing different brands and models of disk, which can have different physical layouts and almost certainly have different numbers of actual blocks/sectors. Putting partitions on the disks ensures that the data is written within sectors rather than across them, and ensures that the head of each disk operates within the same physical boundaries regardless of each disk's physical layout. All disks thereby act the same, ensuring that no one disk can serve as a performance bottleneck on the rest of the pool. But again, I have no idea where I saw that. I might be crazy.
