The animorphs would be as good as a rusty paperclip without Ax. First, there would be only four Animorphs fighting for Earth. Talk about depressing, four kids saving the planet? That's worse than six! Ax is the reason why they get on a yeerk ship and see Visser One, hence Marco's stay. Then, they would be pretty much dead when Jake gets infested. Ax was the first to notice, and even then, the others doubted him. So the yeerk got all the information on "Andalite Bandits" and sold them all out. Goodbye Earth's main protectors! Have a nice afterlife; if that. Third, if somehow Jake does get un-infested, then when they find out what is in Zone 91, how are they gonna figure out it's a toilet? Exactly. And, the Animorphs would die in Z-Space without Ax's quick thinking. I could rant on and on about this, but I won't. Everybody helped with saving the planet, with local (as in human) things, but Ax did any and all "alien" stuff, plus a few other things.