There's another possibility that you guys haven't really addressed. The fact that any "system", which is historically successful, can turn out to be an absolute dud in the future, and usually does.
This is simply because past results can't guarantee any future results, unless there is an underlying connection between the "system" and the horses winning. Invariably there is not, as most of these "systems" are invented by sifting through masses of old race results, trying to find a common thread among the winners.
I can understand why people attempt to sell this crap, but I can't understand why people fall for it.