I know that Bulletproof doesn't fix Metagross' current problems, but that doesn't really seem to be the point. Rather, it lets him be a lot more effective in spite of those problems. Immunities to Shadow Ball, Aura Sphere and Focus Blast are the crux of the buff, letting him single-handedly take on a large number of Pokemon that rely on those moves for coverage or STAB. While the other immunities aren't as big a deal seeing as Metagross already resists most other ball and bomb moves, they can come in handy once in a while (if you're ever up against a Sun team utilizing Weather Ball, I guess?)
Considering Shadow Ball is the most common ghost move in the meta, Metagross would see a raise back to OU.