According to Tom and the odd interview I've seen with Kerry they are open for Gary to contribute but it's this 'business' thing they have to keep sorting out. My guess is it's related to who gets what percentage of everything and how such things need to be split which would still include Jeff and Dave but to a lesser extent than they once were. If contracts haven't been re-written since the early days I imagine there is a lot to deal with. Metallica and Megadeth played together before the big four concerts didn't they. To a lesser extent the NWOBHM is like the Big 4. NWOBHM keeps getting regular mentions these days I don't even know what bands are classified in that now. I think there was also a German Invasion or something at one stage too. I think at the time of the BIg4 everyone expected Mustaine to be the problem, and while he made a few "mustaine-isms" the reporters weren't getting enough so they pushed other bands for stories. I remember something about Lomardo not being happy and that may or may not have been the beginnings of their issues with him but I don't remember much about it all.