BBB is only really useful for those people who want to dispute something. Even then, it's only really useful if that person has tried resolving the issue with you first, and still feels that there's an issue. In other words, the BBB is only going to report negative things about business, never positive. Therefore, the best possible outcome that a potential customer is going to see is a "zero" rating, meaning no one has gone through the BBB complaint process. I don't see much value in that, since there are LOTS of businesses that do subpar work, yet their BBB rating is perfectly fine, because either the issue was remedied or the customer didn't bother to think about or use the BBB.
In other words, you're going to pay the BBB to report that no one has complained about you, that's about it.