Couldn't you already just plug a fairly cheap USB dongle in your rasperry pi ? I'm not sure I see what the big deal is, aside from probably making the price cheaper and freeing up a usb slot
Raspberry Pi with built in wireless comms means it uses a standard piece of hardware so the software stack can be configured to work out of the box. That can't happen with the existing arrangement. Also the power supply problems with original Pis are frustrating to say the least. These are most evident when plugging in USB wifi devices. Again having a standard integrated piece of hardware will (hopefully) avoid this.
With the early RPi it was a bit different because of USB power problems and kernel bugs, but those are mostly solved.
At least, that's what I hope they're doing with it.