I didn't say "increase disk utilization with Swift", I said ZFS does not use the whole disk when you add a new large disk unless you replace the entire vdev worth of disks. Swift treats each disk individually and uses weighting to deal with heterogeneous disk sizes which is much cleaner.
Replacement of entire disks to grow the pool is simply a different way of adding capacity, because ZFS is based on different paradigms than what one is used to with run of the mill implementations; however, in practice, it's a much simpler and therefore much more reliable way to extend capacity. The units are simply entire larger disks.
Swift is also how many orders of magnitude more complex?
