That's the disadvantage I see. You'll need a good few of these to make any significant earnings and that brings us back to Kano's comment about messy USB hubs.
I think the USB sized device is not a good idea, but I think the technology behind it is fantastic. I'm just looking forward to something a little more .... excessive
A device like this isn't about "making significant earnings". If you want significant earnings, you spend signifcant bitcoin and buy an Avalon or a full-blown ASICMINER tens-of-gigahash miner, or a BFL rig (if they ever ship).
A device like this is for someone who believes in Bitcoin, and is happy to drop $30 and 2.5W on "keeping the network honest". Just like all the people out there mining with low-end GPUs at 200MHash/sec on non-BTCGuild pools, because they believe it's good for the network.
Plus, you combine a device like this with a secure hardware wallet and you have an easy to carry full blown bitcoin solution.
A device like this to me seems a good next step towards the mass adoption we're all (?) hoping for.