The PCGA claims that this isn't a big setback -- despite this and a few other losses, they say their numbers have grown, and they cite a few other big still-members, including Microsoft, Nvidia, and Intel. But given how much of an influence Activsion-Blizzard is in PC gaming, it's hard to say you're the "authoritative voice" of the platform when you don't have any formal connection to the biggest developer/publisher in the industry.
What does this mean to us players? Probably nothing right now -- the PCGA is right: the loss of Activision probably won't affect their work at all. But Blizzard, for better or worse, is being steered by Activision away from the industry at large. Right now, with events like BlizzCon and a huge reputation of their own, they don't need to be tied into these industry groups. But that may not always be the case.