While browsing the spam tokens reported by SpamBayes, I realised that spam messages often have the same words, including typos and deliberate mispellings. As described in Paul Graham's A Plan For Spam, it's pretty straightforward to detect spam once you have trained your spam filter. Spammers are victims of their own dubious success; the more spam they send, the more duplicates each person is likely to receive and hence identify automatically as spam. Unfortunately, not everyone knows how to run their own spam filter. Heck, I've been online for years but only just started my own filter.
Julian asked why spammers think anyone would buy mortgages / fake watches / online diplomas / genital enhancements / medicines from a stranger based on a tacky and badly spelt message. Other than the obvious response that enough fools actually respond to spam to keep spammers employed, perhaps enough people accidently click on the spammer's link to earn them advertising dollars!