I didn't intend those Michael Pollan references, but I'll own em :)
I'm not a world traveler by any stretch of the imagination, but I haven't come across a food item that provokes more heated, passionate debate than the venerable Ju(i)cy Lucy. It started out as patrons of Matt's vs. those of the 5-8 club, and as the popularity of the sandwich has flourished, the debate has spilled over into more gourmet interpretations of the burger by places like Blue Door and the Nook. I've even seen it show up as a special at higher end restaurants like Muffaletta.
Part of it is that people like to stand up for places in their neighborhood or places where they have had a positive experience. But from a food snob perspective, I think one of the biggest reasons is this:
As a burger, the Ju(i)cy Lucy is inherently and irreparably flawed.
Stuffed burgers have existed all over the midwest, and they usually suck. When you put cheese inside a burger, it's not going to melt properly unless you cook it to the point that the meat is dry and overdone. So you either have a properly cooked piece of meat with an unappetizing, chewy puck of cheese on the inside (Blue Door), or properly melted cheese-like material inside of a overdone beef patty (Matt's).
In my opinion, the argument is really over which philosophy more effectively rescues the burger from that inherent flaw. Even though I usually abhor American cheese and non-artisan buns, I would argue that the version at Matt's is most successful here; the molten core rescues the dried out beef around it by providing flavor and moisture. On the other hand, the cheese core of the BDP Blucy's detracts from the quality of the burgers - they would be much better burgers if the cheese were properly melted atop the patty.
That (and the blue-collar armosphere) is why Matt's is such an icon, in my opinion. However, I'll include the caveat that if you want a really great burger, don't order a JL.