yeerks have one ability they never exploited to their fullest, they can gain experience, insights, philosophies of those they infest, They can learn faster look at different point of views of different hosts, gain many insights by infesting different hosts, had they done that the animorphs probably would have been captured, their strategic prowess would increase, they would learn about compassion, freedom, and many human qualities that contrast with the empire's way of thinking and life. Their ability to infest and gain knowledgde plus insights would be useful in defeating the andalites, making creative inventions and other stuff, they could surpass the andalites intellectually.