I agree with both CosmicDebris and (surprisingly) sabretoothe. I initially thought that Ussop should have taken on Jyabura, and the victory would send the message to himself and his crew that he was as capable as the rest of them, as he had wanted. Now I realize that such a thing couldn't happen, or else Ussop would stop being the character we love so much.
As I had mentioned in another thread, Ussop is always trying to be something he is not. He wants to be the ideal brave warrior of the sea, he wants to be the captain of the Going Merry, and he wants to be a strong and capable fighter as the rest of his crew. He sets up these goals because he is uncomfortable with who he is and what he is capable of doing. He doesn't think his crewmates appreciates his role on the ship because he sees himself as a weakling that constantly needs to be saved. That's why he pursues difficult challenges to make up for his inabilities, and when he inevitably gets knocked down, he feels humiliated. He keeps pushing to complete his impossible goals because he wants to change himself, not realizing that his crew like him and need him to be the person he has always been.
Winning over Jyabura would have significantly changed his attitude on that aspect. If he was able to take on such a strong opponent, he would have accomplished one of his goals. He would begin to think he was as worthy a fighter as Luffy, Zoro, or Sanji, and start to become more like them. That's not Ussop. A change like that would be so dramatic that he would cease being the character we know. That's why his revelation after Sanji's words is so important. He now realizes that he doesn't need to reach that goal, that he needs to be himself, and that his crewmates would still appreciate him. That is a huge and very important change to his character for him to see what his crew and his many fans have already known about him.
He will do something big by the end of this arc. And even though Spandam is weaker than a Marine, he is still the mastermind behind this whole operation and taking him down would be a very dramatic scene.