If Marco ever went on any missions? He'd definitely prefer to go with the other Animorphs, not some random crewmember. The Animorphs know how to work together, and if it's just them there's no risk of anyone else getting in the way.

Not that he would tell Kang that. "Hey, your tail would be safe with me, my scaly friend," he said, with a grin that probably wouldn't reassure Kang at all.