He also says that this is a clone of what md raid already does, which implies that it suffers from the RAID write hole.
Alsi ability to reshape could be rarely used but you see, when you have to reshape many TiBs, it would be great option to have. Just because it's very troublesome operation otherwise. And btrfs seems to be really flexible when it comes to allocating things, one way or another. And Sun always preferred to publish far more of colorful marketing bullshit than actual design worth of. Sure, they had a business, but they've achieved that now I'm take their achievements with a grain of salt.