Wow! I'm really surprised to read that so many states only require emissions testing in metro areas. Here in DE - it is mandatory statewide - however, not yearly - only when renewing your registration. When buying a new car off the lot, you can get a 5 year registration, so no testing required till that needs to be renewed. After that you can get a 2 year renewal registration, so you have to have the emission check every 2 years.
With all the issues with air pollution, you would think that it would be a requirement that all cars statewide in every state, would have to pass an emission test.