Expresscard pipes directly into your computer's fast pci bus. For video capture, it allows you to capture into a completely different bus than other peripherals, greatly eliminating the chance of capture problems. 4 g broadband, I've read (somewhere in this forum), will also exceed the speed of usb 2 -- so an expresscard is needed to take advantage of quickly coming 4g.
This is not entirely accurate. At least in the case of the 15" laptops that used to have ExpressCard, it depended on the driver whether it would route the esata connection to the USB2 bus or directly via PCI-Express for higher speeds. My roommate in college had consistent crashes with his and we determined beyond a reasonable doubt that the only driver that worked with the controller in his external drive was written to only utilize USB2...and it would kernel panic randomly from sleep/wake actions.
That's a bummer, Tegeril. lasuther, the beef many have with being forced into the 17" is that it's like hauling around a ghetto blaster. But, hey, to each her own!