Ofcourse she's useful.Tsunade is capable of going into combat herself and also supporting others. Her strongest point is her medical ninjutsu which is always useful in battle, Especially since she's praised for it.
If it wasn't for her the 5 kage fight wouldn't have lasted as long as it did. Besides her own tremendous prowess she has a monster summon which can tank even Chou Shinra Tensei, And also (Like Tsunade) Heal anyone she's assigned to heal by her summoner.
She has knowledge over almost everything. If that doesn't make her useful than almost every NV character isn't.