It's not about playing it so much as it is preserving its original shape. If I bought a car (which for that price, you could), I don't want someone to take the engine block out to prove to me that all the parts are original. You're not buying it to play it, you're buying it based purely on its historical and collectible significance. If it were "just the case" that was most important, anyone could take fake a cart body and throw Tecmo Bowl in the guts and call it authentic.
We're talking about 3 screws here. Seriously guys...3 screws. I fail to see how removing 3 screws takes it out of the realm of "historical and collectible significance". I'm willing to bet that you could replace the NWC screws with some Duck Hunt screws and there would be absolutely zero difference.
Actually, anyone could buy a repro cart, paint a case, swap out the guts of said case, print a sticker and pawn it off on some poor soul as legit. Someone could also do the same with the guts of an actual NWC cart and a repro of the game.
Also, if you do happen to purchase one of these carts someday, who's to say it hasn't been taken apart in the past? According to your logic, if it had been taken apart even once it's not worth it? Good luck finding one that comes with that guarantee.
I really don't get grading items but I'm pretty sure it wouldn't have an 85 grade if it didn't work.
EDIT: You guys do realize that if a museum was showcasing an item like this that it would more than likely be taken apart & displayed in a similar fashion? That was this guys intent.