How important is roof felt?

You are right about that . The manufacturers will not warranty the product if it wasn't installed properly . The reason for putting down the felt paper is that your roof sweats from changing attic temps during the course of the day , and on extremely hot days when you are running your AC on high to keep cool , your roof aculates moisture droplets and the felt paper keeps these droplets from soaking into and rotting your roof decking , and about the nail holes in the felt paper , when the roof heats up the tar coating of the felt paper seals itself