Brucey wrote:the thing is that the trickle down of technology means that you can often buy a bike that is 9/10s as well made as a top of the range model was ten or fifteen years ago, but at a much more competitive price. Eventually certain types of technology are consigned to the bargain basement and may be manufactured so poorly that they are not worth using any more. So for example most hubs for screw-on freewheels (and indeed the freewheels themselves) are so poorly made these days that they are not worth having. And whilst 8s and 9s parts work about as well as they ever have done, you can't easily buy swanky XT cassettes in 8s and 9s any more.
Not sure why you'd want an XT cassette, particularly. A bit lighter? The main difference in the road groupsets is the weight, particularly the hollow crank arms on the 105 cranks and above. This is only 200 grams lighter, however, hardly a significant amount. You can probably save a bit here and there on other components as well - more plastic bits on the rear derailleur at 105 and up makes it a bit lighter. The Ultegra shifters uses carbon fibre instead of aluminium, which should make them a bit warmer in the winter?