It's a matter of leading edge tech and salespeak. If you can advertise that your 1000cc bike weighs less than your competitions bike, it sells. Just like advertising more hp or top speed. The other thing is if the bike actually does weigh less, it will perform better, less weight=more hp and better handling etc. Thats why you don't see Duhamel on a 475lb 600RR, even the Buells would be beating him then
The downside to this is decreased longevity and less crash-worthyness. Compare the bodywork thickness of a mid 90's sportbike to a current gen bike. It's probably half as thick. We see F2/3's crash and are back on track next session. We see current R6 and G1ks lowside and have the frames bend/break. The price of "progress".