The one reason I can think of is because they would rather hold their value in bitcoins instead of dollars before they are spent.
Exactly.
If accepting bitcoins draws in customers for a merchant then bitcoins will be a payment method considered.
There are online gamblers, for example, who are winning (yes, some do win) and who aren't planning to cash out via an exchange. They will spend bitcoins at merchants.
Another angle ... merchants can easily sell USD gift cards online in exchange for bitcoins.
