You guys are kind of forgetting, professions are only there to give you things to craft. They don't give you any other type of bonus.
These professions all deal with giving people a bonus on top of their other professions.
They could make something that involves crafting items from multiple other professions, but only the lower quality ones.
Like a profession based solely around making ammo. It would make ammo for both small, and big gunners, and even Micro Fusion Cells, but it wouldn't be able to make the weapons to go with them.
Or hell, better yet, that profession would be able to make ammo in bigger batches, or use less resources.
HOWEVER, that's hardly a real suggestion I was just saying, it shouldn't be something you would use after you take 3 levels as a big gun crafter just so you can get another bonus. It has to be something people may actually use, and not just make alts for.