Random wieght in things like a seatpost and a stem does not do a whole lot, unless you are climbing up mountains. Rotational weight in your wheels effects a whole lot, no matter what you are riding. And lightweight does not equal weak. If you want to show off that your bike only weighs 15lbs. go for it, but function dictates that take all the money you are putting in a new fork, bars, stem, seatpost, ect. and invest in a wheelset.