This is all done so Kallen can eventually kill him. Remember how he admired her piloting? Well, Kallen needs to kill someone in order to be taken seriously, and she's definitely not going to be killing Suzaku or even the other Knights without anymore aces up her sleeve. His tuberculosis mech makes the perfect throwaway character; when he dies, Kallen fans will say, "Look how awesome she is, she killed him," and Shinku fans will say, "He only lost because he had an unexpect coughing fit, let's draw pictures of him and Rolo having sex."
The writers are idiots.