Well, my discussion is specifically about transported troops, not 'ship's troops'.
Transported troops are, yes, 'cargo' in a sense, but they still need to be delivered to the deployment area in condition to fight.
Ship's troops are Marines or PROFORS trained Army troops part of the ship's complement. They're trained in damage control, act as masters-at-arms providing security for the ship, and man gunnery stations aboard some ships. While they are available for deployment at the captain's discretion, such deployments are usually very short term.