I've thought BRICS would use BTC as their main currency especially to avoid sanctions and whatnot. But now I see that won't be the case with their plans to make a currency of their own backed by Gold.
As much as I like bitcoin but it is not yet ready for mass adoption, the price of it is too unreliable for such a large long term commitment like what BRICS countries are making. Imagine if you buy a million barrels of oil (what oil tankers usually carry) worth $80 million in bitcoin and the price drops by 10% which is a normal thing in bitcoin. That is $8 million loss. That's not favorable.
However a centralized currency created and controlled by them is a lot more reliable because they dictate its value (exchange rate) so it can be kept fixed.
At this point bitcoin is good for small size trades and as a small portion of the reserved assets of the country.