Feltup...I've seen Shimano components evolve first hand from the initial launch of the first index systems in the early to mid-eighties to the current day. I can tell you first hand that Dura-Ace components last longer.
I pay for my own components myself and have a combination of 105, Ultegra, and Dura-Ace on my road bike. I have a variety based on the importance I place on the individual component. The rear derailleur being at the top of my list.
I to know many riders with full Dura-Ace equipted bikes that sit around as well. Thats OK, some people like to have nice things for the sake of having them. Heck, one of my co-workers just bought a Porsche 911 convertible and he never 'gets on it'. (I get on him about that!!) Its what he wanted, he enjoys it, and thats all thats important.
Remember, some people just want to 'Be like Mike'.....or Lance, or....
Its all good by me.