One main reason an "exchange" exists in a one place is: speed.
If a buy-sell takes more than few seconds (or milliseconds), why bother with the concept of exchange at all?
Exactly. Have you tried trading on Mt Gox recently?
On secondary points: Think about why an exchange exists: You trust your Bitcoins to the exchange. A p2p exchange is founded on mistrust. While it works for Bitcoin as an absolute currency, mistrust kills it for an exchange.
Bitcoin is founded on mistrust of central authorities. If I'm forced to trust one (like Mt Gox), then that's a bad thing.