This question comes up frequently. The best thing to do if you want to buy something for $500, is buy $500 in bitcoin and then pay with bitcoin. This way your 'stash' isn't eroded and you contribute to the bitcoin economy.
Then what you're proposing eliminates the fundamental purpose of using Bitcoin over existing currency. Why pay transaction fees to an exchange if I can pay zero fees with cash or use a service like Dwolla in which the fee is significantly cheaper than an exchange fee?
Because the 3% credit card fee is already baked into the price of goods you buy. You just don't realize it because merchants are not allowed to visibly offer a cash discount or charge a credit card fee.
Because you want to support bitcoin. If you don't want to pay with bitcoin, don't. If you do want to support bitcoin but don't want to erode your savings, then do this.
