Even here in Illinois where alcohol can't be discounted, the program is great. It's even better when I travel to places that can discount pints and even allow happy hour.
I'm not sure whether discount participants need to be BA members or if any pub/brewery can sign up for the program. I'd guess some pubs don't join the BA since they don't perceive a value in membership so may not have heard of the discount program. Others may be BA members but choose to not participate in the discount program. I have also seen companies with multiple pubs (such as Gordon Biersch, BJ's, Ram, or Rock Bottom) open a new place that takes a while before it shows up on the discount list. I'm not sure if the discount is available from opening or if they have to individually choose to participate.
I guess the one thing to keep in mind when attempting to use the discount is to be polite. Not all staff - or even the on duty manager - may know they are participating in the program. Having a printout probably helps but it's always best policy to graciously accept their decision and then bring the issue up with Kathryn, the pub brewer and/or the pub GM. Some pubs have dropped out of the program because people came in trying to abuse the system; either trying to get the whole party discounted or demanding something other than the discount that pub offers.