I'm not a weight weenie particularly, but there are some times when a lighter bike is nice. I've just started riding my road bike, which is a Cannondale R800, weighs in at 19 pounds and change.

There's a tremendous difference to my hybrid. I really notice it on the few hills we have here, and against the wind. Granted there is a tradeoff in comfort, although I'm still experimenting with seat position, and will change the saddle, but there's never a free lunch. Also at my "advanced" age the lighter weight is more of a factor. Depending on the ride I'm doing, one bike is better to ride than the other, it just comes down to personal preference and the particular situation.