There's actually absolutely no reason for Nami to fight. She doesn't want to fight, she wants to get rich, navigate, get rich, draw maps, get rich, beat variable weather conditions, get rich, and see the whole world while getting rich.
In theory Usopp should want to fight since he's always telling his dream is to become a brave warrior of the seas, but that's not really his basic nature. He's a joker and comedian. He has undeniable master sharpshooter tendencies, but anybody can see he's not actually enjoying the prospect of fighting like Luffy or Zoro. In fact even the other tough guys like Sanji and Franky aren't really looking for fights. Sanji is a cook and wannabe conqueror of women, Franky is an engineer. Robin is a scientist and explorer. Brook is a musician. Chopper is a doctor.
All in all unless the situation forces them all to fight, like it oft has, it would be sufficient if only Luffy and Zoro got really tough guys to beat, since those two thrive in battles. The others have their own diverse interests that aren't blood and gore. The story hasn't altogether forgotten that ever, which is great.