I don't understand the "extra mile" promotion.

As I read it, I could spend $100 to get
up to 10,000 extra miles. I would only
get those miles if I flew a corresponding
distance -- very likely but not guaranteed
should something happen (health, job, whatever). If I do fly that much, I'm buying miles at 1 cent/mile. I hear "rules of
thumb" that value FF miles anywhere from
about 1-2 cents/mile. I'd like to be getting
a bigger discount before I commit in advance
to something that saves me 0-50% at some
point in the indefinite future.

Am I missing something, or is it just that some people value these miles at enough over 1 cent/mile to be willing to pay $100 up front?