Aren't the exchanges only necessary initially, for some definition of "initially"? Surely the goal is that, eventually, enough people are mining (or even receiving bitcoins as payment for labor) and trading among themselves for goods/services without any other currency in the picture.
There will always be other currencies, and currencies that are not exchangeable are less desirable and therefore less valuable.
Do you care if you are paid in Chinese Yuan or USD? The currencies are interchangeable, but Yuan is very hard to convert it USD and you'll lose a lot when you do.
Regardless of how big bitcoin gets, everything will not be transacted in bitcoin. People try to accumulate capital to buy big things, like cars and houses. If you can't buy a beach house in Costa Rica - and any beach house, not just from a few sellers - then the currency is less valuable and people will prefer fiat currencies.
Bitcoin must be easily exchangeable to be widely adopted.
