[rincebrain]

I don't think that's true.

I don't use raidz for my personal pools because it has the wrong set of tradeoffs for my usage, but if I did, I'd absolutely use this.

Yes, your data has the old data:parity ratio for older data, but you now have more total storage available, which is the entire goal. Sure, it'd be more space-efficient to go rewrite your data, piecemeal or entirely, afterward, but you now have more storage to work with, rather than having to remake the pool or replace every disk in the vdev with a larger one.