the important point is that turning USD into bitcoin requires a bank transfer (BT) of some sort. (Out of curiosity, are there exchanges that accept credit card?).
Well, [ ... ] there are other ways to obtain bitcoins that don't involve a BT. Just to name a few: localbitcoin (peer to peer exchange), ATM (Robocoin/Lamassu), mining (as unfeasible or expensive it may seem, it's still a valid method), donations (remember the "Hi mom, send bitcoin here" sign a few months ago?), or even salary (yes, there are employers who offer this opportunity to their employees).
The ATM is another layer between you and the banks. Namely, the cash is deposited into the ATM company's bank account, and transfered to the bank account of some exchange in order to buy moer bitcoins. The fee of that BT is included in the ATM fee.
Granted that there need not be any BT for localbitcoin transactions that are paid in cash; but that requires meeting the BTC seller in person, which is not the most convenient thing in the world. (I read an amusing tale of such a transaction, somewhere on this forum, that took several hours and ended with the buyer feeling obliged to help the seller load some heavy furniture onto a truck.)
Mining and donations would fall into the cases of "have BTC", rather than "have USD".
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In general, payment from USD in your bank to USD in the merchant's bank via bitcoin cannot be cheaper than paying directly in USD, since the latter is just
one BT, the former is
two BTs and other stuff in the middle. The bitcoin path may seem to be cheaper only because the second BT is hidden in the Coinbase fee, and the first BT is paid beforehand and thus does not feel like it is part of the payment.
Actually this is true. But can you pay with dollar bills straight to dell? No not on their website, obviously. You must have a bank account together with a card. And for that it will be fees. Also if you would like to move your USD around the world it will be fees and lots of restrictions. Bitcoin is free!!!