I was thinking about this last night....
First of all, it is right that the Commonwealth nations have some privaliges over non Commonwealth nations. Sorry, but a) there was a war to break away from this, and your forefathers knew there would be pros and cons down the line, and b) Commonwealth countries have had comparative legal systems and administration to the UK over the past several hundred years, so it is likely that a Candian will have been taught to a similar method and standard to a UK national. (I'm not saying BETTER, just similar).
Secondly, as has been said, different States administer drivers ed in different ways. If US drivers ed was made into a Federal system, then I am sure that there would be a good argument for changing the rules, but as it is it would be the same as the UK accepting licences from 51 different countries in one go. It's not practical.
Finally, I thought about the immigration implications of driving without a valid licence. I would say that this renders your insurance invalid. Driving without valid insurance is a criminal offence. So, regardless of how hard done by you feel, to drive in the UK on your US licence over the 12 month mark may have the knock on effect of a criminal record, which would (needless to say) be a MAJOR problem in getting ILR and naturalisation.
Vicky