We've been a member for several years
It's good if you're good. It's a way for client to check on your reputation, your past history of how you have been doing business or if you have had any complaints. The BBB doesn't give details to someone asking about you, just that you are in 'Good Standing' or not with them.
On the other side, if you not a member, they can't report anything about you. Being a 'member' means you have agreed to 'subscribe to the principles and services of the BBB' and will address any and all complaints received by them about you or how you have conducted business with clients that may have complaints in the past.
So again, it's good if you're good.