transferring 4gb with usb isn't so bad compared to transferring a possible 60 gb with usb. firewire definitely seems to be more stable than usb, and sometimes faster, so copying over your entire music collection using usb might frustrate some folks.
you also wouldn't be finding yourself wanting to put a copy of your favorite movie on your nano; they don't play video and a movie would take up a fair chunk of your space. since the new 5gs can play video and have large capacities, transferring whole videos is now an option and having to do that with usb instead of firewire might take longer.
time is such an issue these days isn't it?
i myself am slightly disappointed because i bought my firewire card for my 4g and am planning on getting one of these new 5gs. although, some of the disadvantages associated with the iPods using usb might have been solved, such as when the firmware was updated it would need charging via wall socket (firewire).