I ignore the "by invitation only" thing because I'm pretty dang sure if I called them up and offered them a check, they'd take me!
There's an article coming up in Cake Central magazine that I wrote about networking and one vendor I talked to said affiliations with orgz's such as the BBB were VERY important to her when making recommendations to her brides. (Sorry ... read the articles for more details!
When my husband ran for public office, someone asked him if yard signs really worked in getting votes. He replied, "I don't think they GET you votes ..... but I think if you don't have them, you will definitely lose votes."
Membership in some orgz's can be like that in an opposite way. Not having the BBB logo on your website may not cause someone to click out and move on ..... but it may make a positive impact on them when they see it on your website.
It is a lot of money to invest, especially if you don't have it to spare, so it becomes a decision each business makes based on their goals, resources and long term plan. (gee, that was anything but helpful, huh?
BTW, yes I was a member of the BBB.