As an old dog (over 57 years old), I am learning new tricks every day. In fact, I've always thought that if you don't learn at least ONE new thing a day, you must be dead already.
But seriously, all of the above posts are correct. The purpose of the parking brake is to hold the car before you engage Park on the gear selector. That engages a "dog" against the gears, and keeps them from turning. If you just move the selector into Park, and let the car roll until the dog engages, then you are putting a HUGE strain on the dog, and some day it may shear. The operative word is "someday" This can't be predicted (it is, after all, a piece of metal with finite strength and longevity), but what we are all saying is, why would you not want to "help" you car last longer?
I know, most of your friends with American Iron don't do it; they've never had a problem, but how many of your friends change their cars every five to ten years, and therefore THEY don't encounter this problem. But I assure you, some future owner (it could be you if you keep the car long enough) will have a "broken" transmission because of the lack of care.
So you are right in the final analysis, you will continue to do what you WANT to do, not what you can't teach yourself to do. And someday your transmission will break and you will blame MB for their lack of engineering, and everyone will nod their heads and agree there is no perfect automobile. But guess what? You could treat your car with the care for which it was designed, and you could avoid that problem.
But then, this sort of advice is similar to all of ours "old guys who've been there, done that" when we say change your oil every 3,000 miles, but let's not go THERE today.....Andras