The top end bikes provide the very best of what is currently technically available and that's important to some.
It really boils down to personal choice, like buying a HiFi, most are happy with the quality of the cheaper ones but some want the very best they can have.
At the end of the day, its what you enjoy riding.
I enjoy riding cheap bikes and fully appreciate experienced riders buying expensive bikes. I do cringe when I hear stories like Martin's where cashed up inexperienced riders spend big on bikes where the focus should be on improving their fitness, as its really the nut behind the handlebars that makes the difference, also an experienced rider will customise their ride to suit their abilities and objectives, where an inexperienced rider is yet to learn what will suit them best.
Martin, I've heard a similar story in Sydney, a guy had a few grand and then his girl friend gave him some more and in the end it was something like AU$6,000 (I forget the exact amount), so he went to the LBS to buy a bike and selected one, then he found out it didn't come with pedals, so he got some $10 flat plastic pedals and a $10 lock. Apparently he is just going to ride it from home to the gym.