This is my take on it: you should post SOMETHING. If you don't post some iteration of your inventory, you're off to a very poor start with your buyer. Almost every single dealer's site stats show the New Inventory page as the #1 searched page... if not #2 or #3 behind Pre-Owned or Specials, which is usually an anomale due to a dealer's advertising focus. Point is, when the average user comes to your website, new inventory is what they're looking for.
Let's try out an analogy. You're in a mall, looking for suits. So you have your choice of several stores, though they're all at opposite sides of the mall. You walk into a store, and they have every size but yours on the rack. Would you bolt out of there? No, you'd ask if they have your size or are getting it in sometime soon. Now if they say they don't have it but will get it in soon, then maybe you leave, and maybe you don't. Depends on how soon you need the suit, and whether you need to try it on, and whether the store employee made enough of an impression that you'd want to give him the business, etc.
But if you walked into that store and they had no suits whatsoever on the racks, I think the most you would muster up is something along the lines of "where the hell are the suits??" before walking down to the other store.
But, in defense of the other side of the argument, I'll refer to the comment from Ryan Gerardi, who I imagine is a "civilian" as far as this matter is concerned...
"but I do know that if I find a vehicle at an MB dealership within a 100 or so miles from where I live, my local MB dealer will have it transferred to his own store for the delivery"
If a consumer understands that this is the case, then there's no need to show inventory online, is there? In fact, the consumer creates more work for himself by rummaging around the net to find the car he wants, only to go to his local dealer for it. Ryan, if you walked into my store and told me that you searched every dealer within a 100 miles to find such-and-such car, and now you want me to get it for you, I would ask you why you spent so much time doing the legwork yourself when you could've just sent in a lead to me and asked me to locate that car for you.
But that's just an argument against Ryan's particular comment. The fact is that most consumers aren't looking in a 100 mile radius, and most consumers aren't as loyal to their local store as Ryan seems to be. Indeed, one of the primary goals of a dealer's Internet presence is to draw from markets other than their own immediate market area, right? So I don't want to hear about Ryan's loyalty to his local store if I'm the second closest dealer to him... I want to hear that he came to my website instead, because it's atop the search engines, and that he sent me a lead that says "do you have any AMG CL's in stock? If not, can you find me a black/charcoal one?".