There are other Google Apps that don't work as great on a mobile device. Try Docs on an iPhone, for example. Also, imagine you need to print out a 30 MB PDF that somebody just emailed to you.
Not enamored with QR codes as a solution, though; I still maintain that the vast majority of Americans have no idea what they are and find them, in general, to be a gimmicky pain in the rear. I agree that what you described would actually be more useful, but also probably harder to do (offline = native app).
Google do already provide a set of one-time passwords for those using two-factor auth. I've already added them to a document on my phone for precisely that purpose.
The QR code is displayed on the unsecure connected computer. Your phone network is used to perform the login, so it very little data.
A logical next step would be an app that can streamline the auth a bit (have your username prefilled from the Android account) and send the auth to Google via SMS (often easier and cheaper than getting started with dataroaming).
I'll be using this in the morning to easily log into all my gmail accounts from work. When I leave work I have a logoff script that clears all my cookies. This logs me into all gmail accounts that I am logged into on my phone without having to log in several times.
I can't see a compelling use case for this. It would be more useful to have my phone generate a one-time password without requiring to be connected.