I think this is just really personal, if everyone's collections were the same, we wouldn't need the Bag Showcase thread! I do think it's nice to have a wide variety of styles/colors/brands, but others prefer to just collect many styles in one brand.
I don't think there's anything you must have in your collection, because not everyone's going to like everything. For me, I would never buy a spy bag, that's just not my taste, and I wouldn't want someone telling me my purse collection was incomplete without it.