Sometimes, a purchase or a sale of Bitcoins in the hundreds can take the market up and down by as much as 5-10% depending on the order book. I'm not sure if anybody has noticed, but Coinbase bases the price on quantity as they need to place a market order and then you depending on Bitstamp's order book state at the moment, the price difference can be huge! How are you gonna explain to consumers that they have to pay more, because they're placing a bigger order?

Since most transactions are on the order of 1/1000 of a Bitcoin, it's useful to rephrase what you said as: around 150 million mBTC are being transacted (on the blockchain) per day.