Just reading about LAS, where it talks of "Runway 07L/25R - Extended to be the second longest civilian runway in the country".
AFAIK, LAS handles very few widebodies, with 3 VS 747s each week being the main visitors. Philippine Airlines also operate an A340 via Vancouver. The vast majority of traffic there is along the lines of the B737's operated by Southwest, with AWA also operating a good mix of 319s and 757s.
Even when a 747 does visit, it is never likely to operate the kind of sector length which would push the runway to its limits, and as the city is mainly a destination, rather than a transfer hub, I don't see it being a major candidate for the A380 either (Ivanpah Valley is earmarked to take these sort of flights anyway).
A super-long runway wouldn't allow simultaneous take off / landings, (unlike, say a super long station platform which might handle two or three trains), so where is the big advantage in having such major runway extensions - is it something to do with the elevation / weather conditions, or is it just about ego (everything else there has to be the biggest, so why not the runway?).