I am very interested in btrfs system and I think it may replace ext4 in the future . The installer has an auto allocate script that is triggered when you choose auto allocate or when you choose to install mageia on the entire disk and it is programmed to install mageia on ext4. However if you are planning to support btrfs in a future release, say 10 when btrfs becomes more stable and widespread, it should show a dialog box asking the user if he wants btrfs or ext4 . If he chooses btrfs it should ask him if he wants rsync snapshot support only or btrfs snapshots as well and then it should give him the ideal partioning in either case.
So the auto allocate script should have more than one option and while making things easy for non technical users. Offcourse it is up to the developers to decide this or not, I am just suggesting an idea . I personally think btrfs is great without subvolume headache. I have also learned that Grub does not work properly on btrfs from personal experience.