I don't see why "cheapest, fastest, most secure" is not enough? I have saved hundreds of dollars in fees.
Say i buy an Oz. of gold from Amagi (or whoever). It is $1350.58 if I used a credit card. If I instead use bitcoin I pay only $1311.37.
If I pay with a credit card the seller has to wait for verification to send with confidence. If I pay with bitcoin the money is theirs within an hour and they know it can't be clawed back.
If I pay with credit card I must expose all the information needed to steal from me. If I pay with BTC the seller does not even know who paid. Which is no big deal for them.
The problem is still why it should be better for the merchant, not the seller. I imagine most of the merchants would need to convert the bitcoins back to fiat, and that would have a fee, which could be higher than visa's or mastercard's fee. On the other hand it would be profitable if the merchant is able to buy the products he sells directly in bitcoins.