The problem I'm having with Coinbase is now they want to control volatility on both sides of the equation: during sharp downturns and sharp upturns. So this means that they will quickly and mysteriously "run out of coins" during the next bubble as well.
Indeed, if the price is rising fast, they will be faced with an "unsurmountable opportunity": hold on to their coin reserve as long as they can, by telling customers that they ran out of coins.
In general, a middleman in any trade will take some of the profit for himself, as much as he can.