We are working on pool device removal, but in the meantime, mirrored pools are a bit more flexible in that you can offline/online, replace, attach and detach pool devices, or add another mirrored pair.
With RAIDZ pools, you can offline/online, replace, and add a new RAIDZ vdev, but you can't attach or detach devices.
These operations will also depend on existing redundancy to support the operation.
Another workaround until device removal is available is to create smallish LUNs rather than large LUNs and then use the pools' autoexpand property to expand the pool size when the underlying LUN is expanded. This should help you manage pool space/LUN allocation.