Regarding weight (better stated, mass), in my opinion the advantages of marginally lighter frames/forks/components are as follows:
1. improved performance not due to mass but superior engineering of high end hardware (frame stiffness/flex included here)
2. a good feeling due to genuine aesthetic pleasure, pride, flaunting one's wealth, getting sponsored, etc.
3. possible margin of victory when competing at a professional or near professional level for significant money or prestige
4. marginal performance improvement due to reduced mass - think about it, how much time/effort/money it takes to lose 8 oz from a bike versus from yourself?