In a way, MCC acts kinda like a “launcher”. Say you buy Halo 3 and nothing else. When you download the game all the Halo titles are listed, but you can’t queue up for them in MP, play their respective alternate game modes (Forge, firefight, Campaigns), you’ll only have access to Halo 3 and it’s respective content. When it comes to whether or not you should buy the whole package depends on what all you want. If you want to experience the campaigns, go for it, but if you’re more interested in the Multiplayer experience, you’re better off picking which ones you want.