What was the main battlefield weapon has changed throughout history, but the sword has never been the main battlefield weapon of the samurai. But during the (relatively peaceful 17th to mid 19th century) Edo period only samurai were allowed to regularly wear to wear the long sword and two swords at once (commoners needed special permission to do so usually given as reward for meritous service) so swords became the symbol of the samurai. One of the things that caused the Southwestern War (Satsuma Rebellion) was because the Meiji government banned the wearing of swords. By then it was clear to everyone swords weren't that useful as battlefield weapons, but the ex-samurai were very angry that, after taking away their land and stipend the, the government was taking away their status symbol.

See here and here for more information.

Also if you look for images of historical samurai artwork instead of today's media representations, you'll find a much wider variety of weapons.