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.
You cannot hope to build a better world without improving the individuals. To that end each of us must work for his own improvement, and at the same time share a general responsibility for all humanity, our particular duty being to aid those to whom we think we can be most useful. -Marie Curie
Exactly. The Straw hats aren't an overly typical pirate crew. They don't loot civilian settlements or ships. They aren't exactly heroes, but they are out there to do what they want to do and help people they want to help. What they want to do is often not fighting. Since their living doesn't depend on fighting, or threatening with violence, there's no reason for many of them to fight if they can help it. That's how it should be and I don't mind if the plot occasionally remembers it. It'd be useless for them to have fancy professions, expertise, interests, and dreams if it was all talk and they were only fighting machines marching from one battle to the next.