Hmmm I remember reading somewhere that this sort of thing is not just to fool filters but to screw up Spam software thats tried to learn what is spam and what is not.
The emails contain so much 'normal' text, often in the form of extracts from books, (I suppose because it is relatively random but still is 'proper' english), that everything looks gray to it.

The only way to block spam effectively is to have a whitelist of senders and that isn't usable for a large number of organisations.