I'm a casaul/fitness rider. Last year I took a bike to Burning Man, and brought home 10 more. One was a 1990 Diamond Back, chromo dubble butted frame. I also volunteer at a Bicycle Kitchen and so have access to lots of old parts. So I completely rebuilt the thing, taking special interest in light weight stuff. I even took parts to my drill press (Seat clamp, quick releases, etc... I got it down to 26lbs and was quite proud of myself. But then it absolutely killed me to add a cable lock, pump, and seat pack tool kit. So was it a waste of time? In one sense it was. It was a fun project and still makes me feel good to show it off and ride it.
So if you have the money, and a lighter bike makes you feel good, I say go for it.