For BitCoin it makes no sense to have a few entities generating credit out of thin air, when BitCoins can be directly generated out of thin air by individuals (well, it's indirectly from electricity). However, such entities would start popping up and create credit if the BitCoin supply remains limited.
I mean, then people could just use Ripple and be done with the whole concept of medium of exchange.
I am not sure about what you mean. It's impossible to have a central credit or central bank in the bitcoin economy, unless we're talking about a very good bank that crushed the competition.