If you use an external drive for backup, as most Mac users I know do, it's much better (faster) to use Firewire.
A great advantage of Firewire is the ability to boot into target disk mode; I don't know whether booting from USB will be supported.
Apple killed off FW400 for a reason. It lacks enough of an advantage over USB2 to matter to any "real" pros and to the majority of consumers. There's nothing stopping Apple from reinstating FW800 in future MacBook iterations, just as they initially dropped FW800 in the first MacBook Pro and then reinstated it in the next revision. If Apple plays its cards right, FW800 could kill off USB3 before it hits the ground.