there are also other differences between Raspbian and Bananian.
The binaries for RaspBian in it's upstream repositories are for the Raspi, which has an ARMv6 based processor. But the Allwinner A20 on the BaPi is Cortex-A7 based with a newer instruction set, and are original from the Debian upstream repositories. So the RaspBian binaries are not optimal for the BaPi (performce wise), and RaspBian is also only an inofficial port from Debian. Raspbian so far works well (on my Raspi) and should also work on the BaPi (I haven't had it on the BaPi), but Bananian is better suited for the BaPi, I also do not know how big the difference between Raspbian and Bananian are (performance wise).
Last but not least also the custom kernel of Bananian promises to make better use of the Banana Pi hardware.