As a former manager and an "email direct-marketing" firm, I should point out that the spammers can increase the amount of complexity/variation in the templates by a wide variety of techniques, including rearranging paragraphs instead of just letters, making parts of the message optional, performing syntactic modifications of the included text,... Each new minor modification starts a research effort on the detecting side. The cost of detecting spam will rise much faster than the cost of generating spam.
If you try to outsmart the spammers with this, you will lose. Complexity favors the spammers.