Lance, I just build up a bike and have a carbon seat post. Aside from the expense aspect and I wanted a full set, I see no technological reason for such unless you're a sponsored competitor. The only other reason is if you want and can afford one.

If you're considering either or choices of where to spend your money, I'd pick other areas where weight can have a more benefical affect. ie: wheels, tires, pedals, crank, etc. The stem would be my last choice.