He created an antidote for Queen's Mummy plague, not sure how that is useless. Also, yes he needed Law's help to cure the PH kids, and Ceasar's help to cure the Minks. But he still did his part as a doctor. Not claiming he's the best doctor. Probably will become that by the end of the series. But for now he's a very competent doctor. No shame in not being the best at 17 years old.
Truth is that Chopper is useless...
I mean even Luffy said that if Sanji was bad cook he would kick him out to Dorry and Broggy.
But then again he took Chopper to because he was a doctor.
Still if you playing doctor role in the crew you has to show yout worthless, but even today Chopper somehow forgot that he is doctor...
So in the end Chopper is useless in battle, useless as a doctor, useless as food.
He is useless.