Ok, a lot of VERY good points have been made on here, but you are forgetting one very important reason to collect your vin when getting service: CASH
You may have heard of a company called CARFAX
this company has over 20,000 different data sources both public (police records etc.) and Private (insurance companies, midas, and your dealerships) with a total of over 5 billion records.
as an employee of this company, I can tell you we pay thru the nose for this VIN specific information.
For example... Unibomber
, your VIN is JF1GG29694G803308
I can tell that on August 13th 2004 with 26,417 miles on your car you decided to get your vehicle serviced. (I am assuming for it's 30k mile checkup)
This is amazingly valuable information to the consumer looking to purchase a used car, a dealer looking at a trade in, a bank checking to see if the Loan To Value ratio is high enough.
not that I am trying to stress the importance of my company, I am just saying there is always more than just the obvious reason and money is always a factor...