Keep in mind, I haven't yet read the entire series, but I'm re-reading book 9 now, and there's a scene where Ax suggests he draws the Controllers away from the compound by giving them an Andalite to chase, but Jake says no, they need him inside to work the technology, so Jake then morphs a wolf to distract them. Why couldn't he just morph Ax and thus still distract them with an Andalite?
And then that had me thinking - why don't they just morph Ax from time to time, to help throw the Yeerks off from thinking they were human? Seeing a group of Andalites running by would throw off all the Yeerks that had the theory that the Bandits may have been human.