My thought in general is that it G+ is most helpful and cost effective at MK and this comes to simple math.
MK has the most rides for which it's not uncommon to have hr or greater waits (JC, POC, SpLM, BTMRR, HM, PP, 7DMT ( ILL$), Space, Buz, Pooh, Mermaid and several of the basic rides like dumbo, carosel, people mover, astro, speedway, flying carpets etc). There is more variation as to peoples priorities there. So with more passes and more wider variation on peoples preferences pass availability is greater and efficacy/cost effectiveness of G+ per individual is greater.
At the other parks there are fewer rides with long waits and peoples preferences overlap far more ( At AK - Avitar( ILL$), Navi, Dinosaur, Safari, and Everest; At Epcot - Remy (ILL$), Frozen, TestTrack, Soarin, sometimes Spaceship, Mission Space, living with the land; DHS - RoR (ILL$), MMRR (ILL$), MF, SDD, TT, RnRC, TSM - and sometimes others like Aliens or Star tours). With limited rides and more preference overlap between people the availability of passes becomes limited far quicker and the efficacy/cost effectiveness of G+ per individual is less.