A Bitcoin is worth whatever someone's willing to exchange for it, be it US dollars or peanuts. The same is true of any other currency or item. To suggest that the exchange rate on MtGox or any other exchange sets the value of a Bitcoin is a very limited view.
But the exchanges are revealing publicly what someone is willing to exchange bitcoin for. The exchanges reveal what hundreds or thousands of people are willing to exchange bitcoin for. The market participants collectively set the price of any market and that means that mtgox gives much more information about the market price of bitcoin than ebay does.