OK, so I thought about the third party parking situation before writing my last post. I even thought about taking parking out of the equation so that we could focus on other elite benefits, but went with it anyway. Here was my reasoning...
Those 3rd party companies negotiate with Marriott for the right to run their parking garages. They obviously do it to make a profit. Marriott could quite simply write into the contract that a certain percentage of cars would be parked for free. If the parking company didn't like it, I guarantee another company would be right behind them ready to sign the contract. That's the brilliance of the free market economy.
And I will even point out, in this very thread, the number of people who've said something along the lines of "it would be great if Marriott offered free parking to elites." It wouldn't break Marriott's bank but would CLEARLY set them apart (in a good way) from all the other chains. The message would be: you want to be valued and rewarded for loyalty? Come to Marriott, where we do what it takes to make sure you get treated better than anywhere else.