When you write someone a check, you can't put down "50 USD worth of CAD".
Same with bitcoin. It is its own currency.
just today i sold some coins, face to face. before the final amount of bitcoins to be sent we had to figure out the exchange for canada, and then the bitcoin rate...it would just be easier if it was there, and its not the same as a check...if a pizza cost 1 coin yesterday, and its still one coin...wow, better be a great pizza...