I have a basic membership that does not include credits. I get all of the promos, except the ones that involve credits. I use whatever "cash" I get from these promos to buy the $0.99 Whispersync classics.
I now have close to 200 titles since I first joined Audible (about two years ago) and have never spent more than $1 on any given title; most of them were free. I use the library for more expensive titles.
Audible currently has a
free $5 voucher promo going on. Check
this post for details.