You know those random spams* you get, claiming to be emails from people you know, bearing riches, wonders and attachments. Why is it that they're always *misspelled*? The most common, Snow White and the Seven Dwarfs from hahaha@sexyfun.net, is a classic example, but I've just got rid of a whole bunch of them (there's an email virus going round that talks about some guy's first humour game that you, the recipient, are the first to try out). If they spelled properly, they'd get far more people clicking on them.
Of course, the main problem about virus writers is that they're impatient. If you write a virus that, it quickly becomes apparent on an infected system, has added itself to a whole bunch of executable files and will soon format your hard disc, it will be detected and removed fairly soon. If you write a virus that spreads very slowly, but subtly, just sits around and does very little that is obvious, but occasionally (and thanks to Charlie ages ago for this idea) changes one digit in a spreadsheet, you could - eventually - wreak far more havoc.
Not convinced? Think annual report. Think Enron.
*: The spell-checker knows about the word "spam", but not the plural "spams". (Well, knew - I've enlightened it since.) This strikes me as remarkably optimistic.
