There are plenty of external drive enclosures that support drives (RAID's) that will far exceed Firewire 400 specs. Hence Firewire 800. Remember that your MBP can access these drives just like an internal drive across the FW connection - it's not just for copying files from an external drive to your internal drive, or vice-versa.
Lots of people working with video really lamented the fact that the first MBP's didn't have FW800.