The USB shield only breaks out the appropriate pins. You have to then program the Arduino to be a USB master for whatever you plug into the USB part on the shield.That is not a trivial task, for must USB (slave) devices.Basically, you are writing a driver for the device you have connected, and you have a limited amount of space in which to do that.
well that's the fun of it all, isnt it
do you lose all the arduino pins that it plugs into
but gain some extra ones due to the general purpose 'pins' on the sheild?