I agree--grand parent's assertion of "standardized interfaces they will be slower in achieving greater emissions and fuel efficiency" is complete bullshit. Citation needed like never fucking before.
I know cars and I like modern cars. I don't do well with carburetors. Give me a fuel-injected, electronically controlled system all day long. Give me a modern ECU that will automatically adjust for barometric pressure and ambient temperature. Give me an ECU that will give me good fuel efficiency and emissions. I race my car, but I do not remove my catalytic converter.
They don't want to expose the dealer interface for reprogramming a car's mileage, VIN inside the ECU or new keys w/o having the two existing keys. That's the entire extent of the security concern with the exposure of this information.
Otherwise, if my car is malfunctioning and I want to diagnose why it's not running right, I am all too often hampered or screwed without the dealer tools.
Simple example first: my car developed a knock in a cylinder. To find out whether it's a valve or a rod bearing or piston-slap, I have only one option: disconnect the ignition connector and the fuel connector, start the car, shut it off, etc repeat for each cylinder. However, the dealer can simply go into a menu and trigger an ON_1/OFF_1 (from 1 to 4) for each cylinder, doing the same thing electronically, faster and safer.
Complex scenario: diagnosing catalytic converter failure (which is very emissions relevant!) If the car is running rough or is down on power, especially top-end power, it's possible that this might be caused by a catalytic converter. However, many other things could also be wrong to cause this. The best you gonna do as a DIYer right now (for a 2004+ Mazda, in my case) is to wait until the computer throws a check engine light complaining of cat converter inefficiency. However, your situation might be right on the threshold of the rather-generous factory allowance for catalytic converter performance. The dealer can simply pull up a page of emission stats the the car tracks, which lists catalytic converter efficiency. If it's near the bottom of the range, especially for a car with lower mileage, then you have a dying converter. You can monitor this setting over time as well (E.g. over two weeks) and see if it degrades. Having access to this information can save you non-trivial dollars in gas mileage (e.g. highway can drop from 30mpg to 24mpg easily), fouled up spark plugs and (albeit small) risk of engine damage. Since most modern cars run spark plugs capable of long term replacement intervals, such as 60K miles, you risk fouling up an expensive set of plugs.
This is the simple stuff too. Troubleshooting your ABS system? You get nothing other than an ABS light. Could it be a tear in wiring to an ABS sensor? If you are crafty, you can solder that up cleanly yourself for pennies. Could it be air in the brake lines that got into the ABS module? Bleed the air for the cost of a half liter of brake fluid ($10). If a wheel speed sensor shat the bed, that's a $75 repair. If the ABS module shat the bed, well that's much more expensive. Going to the dealer to find out what the ABS light is all about? You will get slapped with a $90 (1 hr labor) diagnostic fee. As a car enthusiast and an engineer, I don't like paying $90 for 5 minutes of diagnostics. What if you go to your local repair shop for the same problem? If they can't read the ABS code, they will have to spend time going down the list of possible things that could go wrong. E.g. all 4 wheel sensors and wiring would have to be inspected (hope the tear is not obvious or not a failed wire inside a connector, where it's not visible!), brakes bled just in case, parts possibly replaced unnecessarily (on a hunch for a common problem). The pure waste in labor that an independent shop has to do wastes the shop's time and your money.