The only advantage I can think of to run btrfs in a partition would be if there is some reason you want snapshots of the partition. There is no performance advantage over ext4 -- if you check the Phoronix benchmarking articles btrfs is still a bit slower than ext4.
Where btrfs really shines, and where I use it, is to make a multi-device filesystem with RAID 0 and RAID 1 characteristics. So I have 2 WD-1002FAEX SATA 3 drives in a btrfs filesystem, with about 800GB of user data on it:
root@imerabox:/# btrfs fi df /mnt/DATA
Data, RAID0: total=846.00GB, used=795.18GB
Data: total=8.00MB, used=7.62MB
System, RAID1: total=8.00MB, used=64.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=2.00GB, used=1.44GB
Metadata: total=8.00MB, used=0.00
It is pretty fast, and a good way to use these drives.