Sakura is very useful, just not in the "knock down all the bad guys with cool moves" sort of way. Her intelligence helped during the war, she created a cure for an incurable disease that could have killed Kankuro and she was the first to pick up on Tobi's technique. Sakura chose to become a medic-nin and by doing so was taught by Tsunade to never fight in battle and always heal others, so her fighting skills are her best defense, but she can't initiate fights.