While they aren't necessary, they could be helpful. I guess it depends on the frame material...steel or aluminum? The carbon seat stays will help you have a bit of a smoother ride. If the bike itself is steel you might not get as much benefit from them than you would if the bike is aluminum, which is a harsher ride than steel. I would say that if you have two bikes to pick from and that's the only difference...give them each a good test ride and pick the ride that's most comfortable. I have one steel with a carbon fork only and one aluminum with carbon fork and seat stays. I can't say that I feel any great difference in how the two feel on my backside.
Just my .02
Stephanie