Testify wrote:Ninja wouldn't have any place on a battlefield. Stop reading naruto.
I'm fairly sure that the 'men of Iga' (pretty much the only group of 'professional' ninja in the Sengoku period) did a lot of their work on the battlefield. They were famous for their scouting and most of them were samurai (although that was a pretty loose category at the time - anyone with a sword and the skill to use it could basically qualify). The later onmitsu were also mostly samurai.
Just like in the West, most spies and assassins in Japan were members of the nobility / warrior caste. Being able to travel freely, a familiarity with high society, literacy and having both weapons and the training to use them were significant advantages in that line of work. Lower-class spies were much more likely to be servants or locals who were bribed to provide information than specialy trained infiltrators.