Wow, not a single mention of fatigue limits in the article and in the comments here, I MUST POST.
Titanium and steel have a property called a fatigue limit[0], aluminum and carbon do not. There's a stress threshold below which steel and titanium accumulate zero wear, making their cycle life effectively infinite. Every bump Al and CF take puts them on setp closer to failure, and that failure tends to be catastrophic. I will never ride bikes made out of the latter two materials, I need to rely on this machine for my life, I don't want it's cycle life nagging me in the back of my mind.
If you want to save weight, take the brakes and gears off. :P