In principle, I agree. If we suspect a potential Mafia member, we reduce their number by 1. If a Courtesan visits the Captain, the overall result is that the town loses 1 person, while the Mafia remains unscathed. However, based on the Courtesan's visit (presumably, Neyra), the town will immediately figure out who to vote for (i.e., the former Captain). If we hang the wrong person now, the Courtesan will still recruit the Captain, and the situation will become much worse.