i dont think gai and kakashi are going to make a huge difference, but they do have a purpose,maybe. kakashi may use ms to take out some jinchuriki, maybe find a way to unmask toby or figure out who he is.
if gai dies: kakashi will get all rampage-y again, seeing as though everyone he knows would be dead.
if kakashi dies: gai will be thouroughly injured, and NARUTO goes on the rampage.
basically i think the senseis are there to reveal some information or new technique or just to die for the story lines sake. if kakashi dies AGAIN, i'll be doing the rampaging as well.