What's the life expectancy of a CF card used as a
primary hard drive,
for say, a mailserver?
Low, very low.
Remember: You have a hundred thousand or so recordings in each byte of
the card. It has a special mechanism to "distribute wear" but it will come
to an end someday. And when you say about no-flash-oriented software, you
are talking about a lot of writes. And it wears FAST!