It might be worthwhile if:
a - you have a useful serial input device (The "stowaway" serial keyboards for Palm etc spring to mind, given the size of the Pi)
b - you have an input device that uses a non-USB connector and which can't be connected as a HID device using any reasonably priced adaptor (think Model M or TSO workstation keyboards, ADB-connected Wacom tablets, etc)
Basically, if you have a device that you absolutely *must* have connected and which can't plug into a USB hub one way or another. Of course, you'll need to know the protocols used by the device, you'll need to protect the Pi's GPIOs on the physical side, (not that we know how many there will be yet) and you'll need to write and maintain a kernel-side driver for the device.
Not easy, in any sense of the word.