To me it's nitrox, drysuit, deep and wreck. Basically the things that are harder to do/learn yourself or that prevent you from doing something (like diving Silfra without a drysuit speaciality or renting a tank with nitrox for an extended somewhat deeper dive (like 30m). PPB is basically useless because just diving more enables you to learn buoyancy (usually) but I took it anyways as a starting diver to get a bit more guidance.
If PPB does not include reducing the lead weight to the optimum amount and fixing trim issues then I agree, it is useless. Most occasional divers I see dive overweighted and sometimes I see big swings in buoyancy. Some of the divers make adjustments to their diving where they have achieved more or less sufficient buoyancy despite being overweighted. I don't believe any of those divers are working on reducing their weight. That takes an instructor in a course or an experienced buddy to help them.
I was surprised to see several posts placing the self-reliant specialty before PPB. Upon completion of the self-reliant course I can see the conversation going this way: "Congratulations on becoming a self-reliant diver. You can now have uncontrollable ascents and crash into the reef all by yourself now!"