I see exactly one use for the device.
Assume for a moment that we have a family - a mother, a father, two and a half kids, and the dog. We're on a vacation (as opposed to staycation), and we're driving to [popular tourist destination] because it's cheaper than flying, even if it takes 19 hours of non-stop driving to get there.
This family has a netbook for the children in the backseat to play their little saved games (perhaps you stuck an emulator on there and are letting them get acquainted with the golden age of gaming). The father has a mobile broadband card that is plugged into his laptop so that he can get at email.
The kids get bored eventually of playing whatever game it is (Earthbound, perhaps), and want to log on to neopets to check on their zafara and their shoyru. You're at hour 8 of the drive.
* Hand dad's much more expensive laptop to the kids in the back seat and hope they don't screw it up?
* Drop the mobile broadband card into a device designed to share the connection, and connect the kids this way over their netbook?
* Powder your hands and recite the Pimp's Prayer, then reach back and smack a bottom or two, telling them to settle down and keep playing those emulated games you got them, even though they [barely understand how to play them|are bored by your choice of games]?
I'd not hand the kids dad's laptop. That's just a no-go. That's why they got the netbook, so they can stay off the big computer.
It's a little hard to reach back and slap that kid on his ass when you're doing 70.
It's easiest to tell the wifey to drop the card into the mobile router and share the connection. Maybe she'll stop paying as much attention to Facebook and actually, y'know... talk to you while you're driving.
(there are other scenarios that are similar to this, as well.)