Cassie was a very important member.
1) She provides the Animorphs with valuable information about animals; she comes up with brilliant ideas to pull of the missions
In Books #3, 4, 5, 9, 14, 17, 18, 21, 24, etc. etc. she comes up with the basic plan of the mission
2) She can read people's thoughts, and predict people's actions
In Book #22, Cassie is the one who cleverly forms the plan to capture and trap David (a traitor), while the others had come up with nothing~~the plan works perfectly and keeps their cover from getting blown
3) She is the most talented at morphing
In Book #34, on a nearly impossible mission-->she morphs from human to osprey, flies high into the sky, and slowly morphs from osprey to human--falling toward the Yeerk pool. Then she morphed from human to whale (all the while keeping the osprey wings), then landed into the Yeerk pool. She morphed back, then to a Hork-Bajir, and found the stolen Yeerk vessel (in the Yeerk pool) everyone was looking for
Aldrea, a great warrior, was astonished at her accomplishment.
4) She has the magic ability to "coach" the other Animorphs to morph back to their regular selves
In Book #3, without her "coaching", the rest of the Animorphs would have been stuck as half-wolves, half-humans (the 2 hours were drawing to the end or had already passed)
In Book #21, she guides Marco and enables him to morph back. Without her help, Marco would have been stuck permantly as a giant flea (the others had begun to despair)
5) She is a space-time "anomaly"
In "Back to Before", she disintegrates the alternate timeline, which snaps the Animorphs back to their normal lives. The undos the mess Jake created.
Without her help, the Animorphs would have died many times. When all the other Animorphs were sick, she successfully accomplished a mission, saved Aftran, and performed brain surgery on Ax. So, Ax would be dead without her help.
Oh, and once she backed out a mission that involved slaughtering hundreds of innocent people. The other Animorphs decided to side with a Yeerk that was “against Visser Three.” In the end, they were wrong, and they would have been killed (gassed to death). But Cassie went under-cover, and pulled the switch. Her moralizing saved the entire team.