The BBB is a cheap form of advertising. Nothing more, nothing less.
If you have a substantial problem with the company, the BBB will demand that the company rights the wrongs. If not? They don't get their cheap advertising. That's it. That is all the BBB can do. The punishment isn't much at all.
With no real punishment, there is little incentive to be a better store than one without the BBB. However, if a company has been a BBB member for many years, you at least know that the past complaints have been resolved. For that reason, if everything else is equal, you might want to consider a BBB buisness over one that isn't BBB (or that has recently been kicked out of the BBB).