This way, the central banks can trade against that big customer's account.
how would that even work?!
even if Coinbase sells user activity to a big evil bank, that imaginary evil still has the "history" not the future and you can't trade against the history. not to mention that the "history" is already publicly available through charts and the trade history the exchange reveals.
additionally bitcoin market may be small but it is not that small so that one "big customer" becomes that important to follow...
to make it even worse, Coinbase doesn't share that much of the total market volume (i think it is about 6% of total volume) to be effective.