Unregistered2

Usopp was the ship repairing guy till Franky came. So he did lose use, but he's far from useless. He's the only one who can use the cannons with exact precision and he has taken to inventing rather than building since Franky came. He made both Nami's weapon, and the salt balls they're using now(which are a lot handier than going into battle with a shaker.) They have already explained that Usopp's strength doesn't lie in his actual strength. As for the dials, he uses flame dials in his weapon, and impact dials would be kinda stupid to use on zombies. His ingenuous use of them on Luffy was because he knows Luffy so well that he was able predict how he would fight and had a day to actually prepare the battlefield.