Funny you should mention the Cobra...here's a personal anecdote that may provide some perspective on the points you've raised:

I was furniture shopping in Jamestown, California last summer and stopped with my girlfriend and her mother to inspect a Cobra parked by the curb. It was "obvious" to my discerning eyes that it was a replica because of the fresh Racing Green Metallic paint, and the fact that all of the metal, leather, carpets and interior parts looked brand new (i.e., with no fading, wear, discoloration, etc. that you'd expect to see some evidence of in a 50 year old car, no matter how well it was maintained). It was a beautifully done car, and visually looked like the real thing, right down to the Hallibrand knock offs, proper gauges, etc. The owner clearly had a lot of money invested in it.

So we're admiring the car, and my girlfriend asks me the inevitable question: "Honey, is that a real Cobra or a kit car?" To which I naturally replied "It's definitely a kit car", and then started to rhyme off the reasons why. By that point, the owner had walked up behind us - he'd heard our entire conversation. He shot me a contemptuous look and blurted indignantly "Oh, you think it's a kit car." He didn't refute or acknowledge whether it was or not, and was so incensed that he didn't wait for me to say how beautiful I thought his Cobra was (replica or not). He stormed past us, jumped into the car and took off, tires squealing. Man, he was soooo pissed...

Why was the owner so incensed? Well, a couple of plausible explanations come to mind:

1. He paid half a million dollars for an original, meticulously restored Cobra 427, and is ticked off that people routinely mistake it for a "cheap" replica.

2. It actually was a kit car, and he was pissed off that all of the money and effort he spent didn't fool us into thinking otherwise.

My money's on door number two, but my point is really that the quality of many of the latest generation kits cars is superb. The industry has come a long way since the eighties, when you could have easily spotted a replica a mile away. Today, the D&R and NAERC 2nd gen replica Diablos are what are referred to in the kit car industry as "exact replicas", and are visually nearly indistinguishable from the real thing. And at $70-$80K to build one right, they're not exactly cheap. If you spend the money to build one correctly and pay attention to the details, just about the only visual cue to its origins while the car is parked is that the brake discs on the replica are noticeably smaller than on an original Diablo - and even that can be fixed by installing larger rotors and Brembo calipers.

I can understand why owners of real Diablos and Murcielagos would be irked with having spent substantially into the six figures on the real thing only to be routinely (and legitimately) asked by casual passerbys: "Gee, cool car mister! Is that a real Lamborghini or a Fiero?" Even if some bystanders don't ask the question, you know that a good proportion of them are wondering it. The existence of kit cars may not directly devalue the originals, but they do definitely diminish the ownership experience of having the real thing.