Model years are much more complicated than simply ways for manufacturers to cheat you out of your money.
Environmental issues -
The EPA in the US issues regulations based on model years. Yes, they could say all cars registered after 1/1/2000 must comply with ..., however, as pointed out here several times, new cars may or may not get registered immediately after being produced.
So would it be right to have a car built to the pre-1/1/2000 requirement, say built 11/30/1999, have to be junked due to it not being sold and registered prior to 1/1/2000? The use of model year eases this problem, gives a clear switch-over in the production for incorporating new regulations, and allow manufacturers to not be penalized because someone didn't buy and register the car quick enough.
Real life case: In 1978, Chevrolet announced the Pace Car model. Many people bought these and trailered them to long-term storage, as they intended these to be investments. However, as many know, the investment dream failed and so many were later registered and driven on the street as regular cars.
Should Chevrolet be punished for building a car built to 1978 standards that is first registered in 1984 or 1988, which of course does not meet those
1984 or 1988 standards?
Safety regulations -
Same situations, each manufacturer has to comply with safety equipment that complies with the standards for a given date. However, the MODEL year is certified, not the model, as some models range over several years and thus several different standards.
Buying New Old Stock -
Unlike the UK where I have seen two and three year old "new" Minis in dealers waiting to be sold, in the US, the model year automatically establishes a car as old when the model year changes. Thus buyers are protected from buying a car that is three years old, even if it has zero miles, for the same price as a new car which was built a week or month ago. There are laws that regulate this.
A little help for Yahoo - UK finds:
The Rover Defender apparently refers to model years as this article on the Defender 03 model year.
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-- Fiat introduce the new Panda "Model Year 2007"
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-- More style, safety and performance for 2004 model year RAV4,
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-- Ka and Fiesta - enter the 2001 model year
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excel History and Classification - apparently, Lotus does use model years.
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and
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So model year is not unheard of in the UK, just not used that much.