I have 2 480s plugged directly on my mobo, and I've had no problem maybe I should don't understand why it would burn up there and not the pci port.
Even if you use the riser cables it wouldn't help in the event of that because the same power would be going to your mobo from your power supply.
Powered risers have a molex power connector that supplies power to the card. All power pins on the PCIE connector are powered through this connector not the mother board.