first factor is the weight. for starters easier to carry get on and off the car etc.
you will definately paddle easier and be able to paddle at a higher speed for longer. in the flatwater thats going to add up for any racer or paddler- elite or novice.
who likes doing it harder

in the ocean on a big downwinder the wood and carbon may even out a bit, but in lighter stuff it would have to be better as per the reasons above.
carbon boards generally have a bit of twang or life about them compared to normal epoxy boards. wood dw board seem a bit 'dull ' even though they might be fast imo
most elite racers like them lite, most surfers like their boards lite, most cyclists like their bikes lite etc etc.
i guess it gets down to the dollars
cheers