Because the usefulness of money is proportional to its liquidity.
If you buy supplies with USD, and sell them for MITCoins, you are critically dependent on a small bid/ask spread, and there won't be if the only holders are MIT students.
Consequently, you would be able to choose to buy for USD at a store, or for MITCoins and pay a markup that is equal to the spread (since the buyer needs to pay the ask (to get his MITCoins) and the seller gets paid the bid (to get USD to buy new supplies)).
If you buy supplies with USD, and sell them for MITCoins, you are critically dependent on a small bid/ask spread, and there won't be if the only holders are MIT students.
Consequently, you would be able to choose to buy for USD at a store, or for MITCoins and pay a markup that is equal to the spread (since the buyer needs to pay the ask (to get his MITCoins) and the seller gets paid the bid (to get USD to buy new supplies)).
This would mean that no trading would ever begin.