I don't really understand your statement/question. Did you see it IN the system? As in with the internals exposed? Or are you simply stating that you've seen a U.S.B. WiFi nubbin installed (plugged in)?
As far as I know, Xbox does not integrate with your standard P.C. accessories. Could you have possibly seen the standard Xbox WiFi U.S.B.s plugged into a slim? That being the case, that would be rather useless. It would be like installing a fifth wheel onto
your standard passenger car: you can do it, but it is completely unnecessary, and won't increase your performance.