Well, you might plan on spending them instead of selling them.  The exchange rate matters then, because it gives an indication of purchasing power.  Spending bitcoins makes about as much sense as spending any other currency assuming merchants exist that offer what you want and accept bitcoin as payment.  The determination of which currency to spend would likely be determined by which offers the higher purchaing power at the moment.  The reduce transaction fees, and fraud risk for merchants is likely to result in more purchaing power when spending bitcoins than other currencies.