I think that you have to be smart when it comes to the type of bike you ride and the components you hang on it. You really need to take into account how much you weigh and what type of riding you plan on doing.
If you weigh 200 pounds, it's probably not a good idea to be riding a 21 pound mountain bike, unless it's strictly a race bike. Also, if you plan on doing 10 foot drops and stunts, you need to be on a strong bike which is going to be heavy. If you are doing 6+ mile climbs, it wouldn't be much fun if you were dragging a 40 pound anchor up the mountain.

Personally, I ride on a fairly light weight full-suspension bike that has a carbon front end. My bike weighs around 24.5 pounds with disc brakes. I'm an XC rider and don't go around doing 10 foot jumps. Also, I weigh a 160 pounds so my bike can take the abuse I put it through. It's a lot more enjoyable doing an 8 mile climb on a light bike than on a heavy tank.

Does this make me a weight weenie? Well, maybe or maybe not. I'm not fanatical about putting the lightest components on my bike but I do want my bike to be fairly light.