But then it occured to me: maybe errors help filter out attentive respondents, who have a low chance of completing the whole scam funnel. Those people who don't notice the errors, or don't consider them a red flag, are better leads.

In the context of a cliche'd Nigerian origin, sure, mentioning Nigeria puts anyone familiar with the 419 scam formula on guard. But anyone who does reply is likely a fresh, naive prospect. Ka-ching!