Yeah, as mentioned in the post, you can indeed just add a vdev to the pool whenever you want. But the risk, apart from performance, is data protection If you add one disk, your whole pool is at risk. So of course you have to add two or more disks as a mirror or RAIDZ but lots of home users don't want to do this. They want that Drobo thing where they throw another log on the fire and everything gets rebalanced. It's sad that ZFS can't do this, and that it can't "upgrade" RAID-Z1 to RAID-Z2. It's hard to do, but would be so much nicer.
